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Aims Our recent study demonstrated that increased Ca2+ sparks and spontaneous transient outward currents (STOCs) played an important role in uterine vascular tone and haemodynamic adaptation to pregnancy. The present study examined the role of ryanodine receptor (RyR) subtypes in regulating Ca2+ sparks/STOCs and myogenic tone in uterine arterial adaptation to pregnancy. Methods and results Uterine arteries isolated from non-pregnant and near-term pregnant sheep were used in the present study. Pregnancy increased the association of α and β1 subunits of large-conductance Ca2+-activated K+ (BKCa) channels and enhanced the co-localization of RyR1 and RyR2 with the β1 subunit in the uterine artery. In contrast, RyR3 was not co-localized with BKCa β1 subunit. Knockdown of RyR1 or RyR2 in uterine arteries of pregnant sheep downregulated the β1 but not α subunit of the BKCa channel and decreased the association of α and β1 subunits. Unlike RyR1 and RyR2, knockdown of RyR3 had no significant effect on either expression or association of BKCa subunits. In addition, knockdown of RyR1 or RyR2 significantly decreased Ca2+ spark frequency, suppressed STOCs frequency and amplitude, and increased pressure-dependent myogenic tone in uterine arteries of pregnant animals. RyR3 knockdown did not affect Ca2+ sparks/STOCs and myogenic tone in the uterine artery. Conclusion Together, the present study demonstrates a novel mechanistic paradigm of RyR subtypes in the regulation of Ca2+ sparks/STOCs and uterine vascular tone, providing new insights into the mechanisms underlying uterine vascular adaptation to pregnancy. [ABSTRACT FROM AUTHOR]

Hypoxia during pregnancy profoundly affects uterine vascular adaptation and increases the risk of pregnancy complications, including preeclampsia and fetal intrauterine growth restriction. We recently demonstrated that increases in Ca2+ sparks and spontaneous transient outward currents (STOCs) played an essential role in pregnancy-induced uterine vascular adaptation. In the present study, we hypothesize that gestational hypoxia suppresses Ca2+ sparks/STOCs coupling leading to increased uterine vascular tone via enhanced endoplasmic reticulum (ER)/oxidative stress. Uterine arteries were obtained from nonpregnant and near-term pregnant sheep residing in low altitude or acclimatizing to high-altitude (3801 m) hypoxia for ≈110 days. High-altitude hypoxia suppressed pregnancy-induced upregulation of RyR1 and RyR2 (ryanodine receptor 1 and 2) protein abundance, Ca2+ sparks, and STOCs in uterine arteries. Inhibition of Ca2+ sparks/STOCs with the RyR inhibitor ryanodine significantly increased pressure-dependent myogenic tone in uterine arteries from low-altitude normoxic pregnant animals but not those from high-altitude hypoxic pregnant animals. Gestational hypoxia significantly increased ER/oxidative stress in uterine arteries. Of importance, the hypoxia-mediated suppression of Ca2+ sparks/STOCs and increase in myogenic tone in uterine arteries of pregnant animals were reversed by inhibiting ER/oxidative stress. Of great interest, the impaired sex hormonal regulation of STOCs in high-altitude animals was annulled by scavenging reactive oxygen species but not by inhibiting ER stress. Together, the findings reveal the differential mechanisms of ER and oxidative stresses in suppressing Ca2+ sparks/STOCs and increasing myogenic tone of uterine arteries in hypoxia during gestation, providing new insights into the understanding of pregnancy complications associated with hypoxia. [ABSTRACT FROM AUTHOR]

Spontaneous transient outward currents (STOCs) at physiological membrane potentials of vascular smooth muscle cells fundamentally regulate vascular myogenic tone and blood flow in an organ. We hypothesize that heightened STOCs play a key role in uterine vascular adaptation to pregnancy. Uterine arteries were isolated from nonpregnant and near-term pregnant sheep. Ca2+ sparks were measured by confocal microscopy, and STOCs were determined by electrophysiological recording in smooth muscle cells. Percentage of Ca2+ spark firing myocytes increased dramatically at the resting condition in uterine arterial smooth muscle of pregnant animals, as compared with nonpregnant animals. Pregnancy upregulated the expression of RyRs (ryanodine receptors) and significantly boosted Ca2+ spark frequency. Ex vivo treatment of uterine arteries of nonpregnant sheep with estrogen and progesterone imitated pregnancy-induced RyR upregulation. STOCs occurred at much more negative membrane potentials in uterine arterial myocytes of pregnant animals. STOCs in uterine arterial myocytes were diminished by inhibiting large-conductance Ca2+-activated K+ (BKCa) channels and RyRs, thus functionally linking Ca2+ sparks and BKCa channel activity to STOCs. Pregnancy and steroid hormone treatment significantly increased STOCs frequency and amplitude in uterine arteries. Of importance, inhibition of STOCs with RyR inhibitor ryanodine eliminated pregnancy- and steroid hormone-induced attenuation of uterine arterial myogenic tone. Thus, the present study demonstrates a novel role of Ca2+ sparks and STOCs in the regulation of uterine vascular tone and provides new insights into the mechanisms underlying uterine vascular adaptation to pregnancy. [ABSTRACT FROM AUTHOR]

Key points: Gestational hypoxia represses ten–eleven translocation methylcytosine dioxygenase 1 (TET1) expression in uterine arteries, which is recovered by inhibiting endogenous miR‐210.Inhibition of miR‐210 rescues BKCa channel expression and current in uterine arteries of pregnant animals acclimatized to high altitude hypoxia in a TET‐dependent manner.miR‐210 blockade restores BKCa channel‐mediated relaxations and attenuates pressure‐dependent myogenic tone in uterine arteries of pregnant animals acclimatized to high altitude. Gestational hypoxia at high altitude has profound adverse effects on the uteroplacental circulation, and is associated with increased incidence of preeclampsia and fetal intrauterine growth restriction. Previous studies demonstrated that suppression of large‐conductance Ca2+‐activated K+ (BKCa) channel function played a critical role in the maladaptation of uteroplacental circulation caused by gestational hypoxia. Yet, the mechanisms underlying gestational hypoxia‐induced BKCa channel repression remain undetermined. The present study investigated a causal role of microRNA‐210 (miR‐210) in hypoxia‐mediated repression of BKCa channel expression and function in uterine arteries using a sheep model. The results revealed that gestational hypoxia significantly decreased ten–eleven translocation methylcytosine dioxygenase 1 (TET1) expression in uterine arteries, which was recovered by inhibiting endogenous miR‐210 with miR‐210 locked nucleic acid (miR‐210‐LNA). Of importance, miR‐210‐LNA restored BKCa channel β1 subunit expression in uterine arteries, which was blocked by a competitive TET inhibitor, fumarate, thus functionally linking miR‐210 to the TET1–BKCa channel cascade. In addition, miR‐210‐LNA reversed hypoxia‐mediated suppression of BKCa channel function and rescued the effect of steroid hormones in upregulating BKCa channel expression and function in uterine arteries, which were also ablated by fumarate. Collectively, the present study demonstrates a causative effect of miR‐210 in the downregulation of TET1 and subsequent repression of BKCa channel expression and function, providing a novel mechanistic insight into the regulation of BKCa channel function and the molecular basis underlying the maladaptation of uterine vascular function in gestational hypoxia. Key points: Gestational hypoxia represses ten–eleven translocation methylcytosine dioxygenase 1 (TET1) expression in uterine arteries, which is recovered by inhibiting endogenous miR‐210.Inhibition of miR‐210 rescues BKCa channel expression and current in uterine arteries of pregnant animals acclimatized to high altitude hypoxia in a TET‐dependent manner.miR‐210 blockade restores BKCa channel‐mediated relaxations and attenuates pressure‐dependent myogenic tone in uterine arteries of pregnant animals acclimatized to high altitude. [ABSTRACT FROM AUTHOR]

Pregnancy enhances endothelium-dependent relaxation of ovine uterine artery: role of NO and intracellular [Ca.sup.2+]. Am J Physiol Heart Circ Physiol 281: H183-H190, 2001.--The present study tested the hypothesis that the pregnancy-associated increase in endothelium-dependent relaxation of the uterine artery was mediated primarily by an increase in nitric oxide (NO) release, resulting in a reduction in smooth muscle intracellular [Ca.sup.2+] concentration ([[[Ca.sup.2+]].sub.i]). Uterine arteries obtained from nonpregnant and near-term (140 days gestation) pregnant sheep were used. The [Ca.sup.2+] ionophore A23187 induced endothelium-dependent relaxations in both nonpregnant and pregnant uterine arteries, with an increased relaxation in the pregnant tissue. In contrast, endothelium-independent relaxations induced by sodium nitroprusside were the same in nonpregnant and pregnant arteries. In addition, removal of the endothelium significantly increased noradrenaline-induced contractions in pregnant, but not nonpregnant, uterine arteries. In accordance, pregnancy increased both basal and A23187-stimulated NO releases in the uterine artery. Simultaneous measurement of tension and [[[Ca.sup.2+]].sub.i] in the smooth muscle demonstrated a linear correlation with the slope of unity between A23187-induced relaxation and the reduction of [[[Ca.sup.2+]].sub.i] in both nonpregnant and pregnant uterine arteries. The A23187-induced reduction of [[[Ca.sup.2+]].sub.i] was significantly enhanced in pregnant, compared with nonpregnant, uterine arteries. The results indicate that pregnancy increases NO release, which, through decreasing [[[Ca.sup.2+]].sub.i] in the smooth muscle, accounts for the increased endothelium-dependent relaxation of the uterine artery. Signal transduction pathways distal to NO production are not changed by pregnancy. calcium

Nauli, Surya M., Lubo Zhang, and William J. Pearce. Maturation depresses cGMP-mediated decreases in [[Ca.sup.2+]].sub.i] and [Ca.sup.2+] sensitivity in ovine cranial arteries. Am J Physiol Heart Circ Physiol 280: H1019-H1028, 2001.--Because cerebrovascular cGMP levels vary significantly during maturation, we examined the hypothesis that the ability of cGMP to relax cerebral arteries also changes during maturation. In concentration-response experiments, potassium-induced tone in basilar arteries was significantly more sensitive to a nonmetabolizable cell-permeant cGMP analogue 8-(p-chlorophenylthio)-cGMP (8-pCPT-cGMP) in term fetal [-log one-half maximal concentration ([EC.sub.50]) = 4.4 [+ or -] 0.1 M] than in adult (-log [EC.sub.50] = 4.0 [+ or -] 0.1 M) ovine basilar arteries. Serotonin-induced tone also revealed significantly greater sensitivity to the cGMP analogue in fetal (-log [EC.sub.50] = 4.9 [+ or -] 0.1 M) than in adult (-log [EC.sub.50] = 4.7 [+ or -] 0.1 M) basilars. In fura 2-loaded preparations, 8-pCPT-cGMP had no significant effect on cytosolic calcium concentrations in potassium-contracted arteries but at 6 [micro]M significantly reduced calcium only in fetal basilars ([Delta] = 33 [+ or -] 8%). Higher 8-pCPT-cGMP concentrations reduced cytosolic calcium in both fetal and adult basilars. Similarly, in both potassium- and 5-hydroxytryptamine (5-HT)-contracted preparations, low concentrations of 8-pCPT-cGMP reduced myofilament calcium sensitivity only in fetal basilars ([Delta] = 29 [+ or -] 6 and A = 42 [+ or -] 10%, respectively), whereas higher concentrations reduced calcium sensitivity in both fetal and adult arteries. In [Beta]-escin-permeabilized arteries, equivalent reductions in basal and agonist-enhanced myofilament calcium sensitivity were produced by much lower 8-pCPT-cGMP concentrations in fetal (172 and 61 [micro]M, respectively) than in adult (410 and 231 [micro]M, respectively) basilars. The mechanisms mediating cGMP-induced vasorelaxation appear similar in fetal and adult arteries, with the exception that they are much more sensitive to cGMP in fetal than adult arteries. These age-related differences in the sensitivity of cytosolic calcium concentration, basal, and agonist-enhanced myofilament calcium sensitivity to cGMP can easily explain why both potassium- and 5-HT-induced tone are more sensitive to cGMP in fetal than adult cerebral arteries. 8-(p-chlorophenylthio)-guanosine 3',5'-cyclic monophosphate; cerebral arteries; cerebrovascular circulation; guanylate cyclase; intracellular calcium concentration

Effects of maturation and acute hypoxia on receptor-[IP.sub.3] coupling in ovine common carotid arteries. Am J Physiol Regulatory Integrative Comp Physiol 280: R410-R417, 2001.--Whereas previous studies have established that many mechanisms mediating pharmacomechanical coupling are subject to regulation, evidence of physiological regulation of the coupling efficiency between receptor activation and second-messenger production is scarce. The present studies address the hypothesis that acute hypoxia and maturation can influence the mass of second-messenger production for each activated agonist-bound receptor ('receptor gain'). For this assessment, receptor density and agonist affinity values were used to calculate 5-hydroxytryptamine (5-HT) concentrations that would produce standardized numbers of bound receptors (8.5 fmol/mg protein) in each experimental group and thus minimize effects of age or hypoxia on receptor density or agonist affinity. After 3 min of exposure to these 5-HT concentrations, normoxic magnitudes of contraction were similar (as %potassium maxima) in fetal (50 [+ or -] 14%) and adult (40 [+ or -] 9%) arteries, but hypoxia ([PO.sub.2] [approximately equals] 9-12 Torr for 30 min) depressed contractile tensions with a significantly different time course and magnitude in fetal (30 [+ or -] 10%) and adult (17 [+ or -] 11/%) arteries (P [is less than] 0.05). Basal inositol 1,4,5-trisphosphate ([IP.sub.3]) values (in pmol/mg protein) were significantly greater in fetal (94 [+ or -] 16) than in adult (44 [+ or -] 6) arteries, and integrated areas above baseline for the [IP.sub.3] time courses (in nmol-s/mg protein) were significantly greater in fetal than in adult arteries both in normoxic (14.3 [+ or -] 1.8 vs. 9.1 [+ or -] 1.6) and hypoxic (15.0 [+ or -] 2.1 vs. 8.6 [+ or -] 1.2) conditions (P [is less than] 0.05). Hypoxia altered the [IP.sub.3] time courses both in the fetus and the adult but had no significant effect on [IP.sub.3] mobilization or receptor gain. These data demonstrate that for the 5-[HT.sub.2a] receptor predominant in this preparation, receptor gain can be experimentally determined, is not influenced by acute hypoxia, but is greater in fetal than in adult ovine carotid arteries. serotonin; intrinsic efficacy; coupling efficiency; fetal lamb; grams protein

Upregulation of eNOS in pregnant ovine uterine arteries by chronic hypoxia. Am J Physiol Heart Circ Physiol 280: H812-H820, 2001.--We tested the hypothesis that chronic high-altitude (3,820 m) hypoxia during pregnancy was associated with the upregulation of endothelial nitric oxide (NO) synthase (eNOS) protein and mRNA in ovine uterine artery endothelium and enhanced endothelium-dependent relaxation. In pregnant sheep, norepinephrine-induced dose-dependent contractions were increased by removal of the endothelium in both control and hypoxic uterine arteries. The increment was significantly higher in hypoxic tissues. The calcium ionophore A23187-induced relaxation of the uterine artery was significantly enhanced in hypoxic compared with control tissues. However, sodium nitroprusside- and 8-bromoguanosine 3',5'-cyclic monophosphate-induced relaxations were not changed. Accordingly, chronic hypoxia significantly increased basal and A23187-induced NO release. Chronic hypoxia increased eNOS protein and mRNA levels in the endothelium from uterine but not femoral or renal arteries. In nonpregnant animals, chronic hypoxia increased eNOS mRNA in uterine artery endothelium but had no effects on eNOS protein, NO release, or endothelium-dependent relaxation. Chronic hypoxia selectively augments pregnancy-associated upregulation of eNOS gene expression and endothelium-dependent relaxation of the uterine artery. high altitude; endothelium; gene expression; endothelial nitric oxide synthase

Long, Wen, Lubo Zhang, and Lawrence D. Longo. Cerebral artery [K.sub.ATP]- and [K.sub.Ca]-channel activity and contractility: changes with development. Am J Physiol Regulatory Integrative Comp Physiol 279: R2004-R2014, 2000.--The present study was designed to test the hypothesis that in cerebral arteries of the fetus, ATP-sensitive ([K.sub.ATP]) and [Ca.sup.2+]-activated [K.sup.+] channels ([K.sub.Ca]) play an important role in the regulation of intracellular [Ca.sup.2+] concentration ([[Ca.sup.2+].sub.i]) and that this differs significantly from that of the adult. In main branch middle cerebral arteries (MCA) from near-term fetal (~140 days) and nonpregnant adult sheep, simultaneously we measured norepinephrine (NE)-induced responses of vascular tension and [[[Ca.sup.2+].sub.i]] in the absence and presence of selective [K.sup.+]-channel openers/blockers. In fetal MCA, in a dose-dependent manner, both the [K.sub.ATP]-channel opener pinacidil and the [K.sub.Ca]-channel opener NS 1619 significantly inhibited NE-induced tension [negative logarithm of the half-maximal inhibitory concentration (p[IC.sub.50]) = 5.0 [+ or -] 0.1 and 8.2 [+ or -] 0.1, respectively], with a modest decrease of [[[Ca.sup.2+].sub.i]]. In the adult MCA, in contrast, both pinacidil and NS 1619 produced a significant tension decrease (p[IC.sub.50] = 5.1 [+ or -] 0.1 and 7.6 [+ or -] 0.1, respectively) with no change in [[[Ca.sup.2+].sub.i]]. In addition, the [K.sub.Ca]-channel blocker iberiotoxin ([10.sup.-7] to [10.sup.-6] M) resulted in increased tension and [[[Ca.sup.2+].sub.i]] in both adult and fetal MCA, although the [K.sub.ATP]-channel blocker glibenclamide ([10.sup.-7] to 3 x [10.sup.-5] M) failed to do so. Of interest, administration of [10.sup.-7] M iberiotoxin totally eliminated vascular contraction and increase in [[[Ca.sup.2+].sub.i]] seen in response to [10.sup.-5] M ryanodine. In precontracted fetal cerebral arteries, activation of the [K.sub.ATP] and [K.sub.Ca] channels significantly decreased both tension and [[[Ca.sup.2+]sub.i]], suggesting that both [K.sup.+] channels play an important role in regulating L-type channel [Ca.sup.2+] flux and therefore vascular tone in these vessels. In the adult, [K.sub.ATP] and the [K.sub.Ca] channels also appear to play an important role in this regard; however, in the adult vessel, activation of these channels with resultant vasorelaxation can occur with no significant change in [[[Ca.sup.2+].sub.i]]. These channels show differing responses to inhibition, e.g., [K.sub.Ca]-channel inhibition, resulting in increased tension and [[[Ca.sup.2+].sub.i]], whereas [K.sub.ATP]-channel inhibition showed no such effect. In addition, the [K.sub.Ca] channel appears to be coupled to the sarcoplasmic reticulum ryanodine receptor. Thus differences in plasma membrane [K.sup.+]-channel activity may account, in part, for the differences in the regulation of contractility of fetal and adult cerebral arteries. cerebrovascular circulation; vascular smooth muscle; sympathetic nervous system; norepinephrine; intracellular calcium; potassium channels; fetus; adult

Longo, Lawrence D., Yu Zhao, Wen Long, Carolyn Miguel, Ryan S. Windemuth, Anne-Maree Cantwell, Alice T. Nanyonga, Tsuyoshi Saito, and Lubo Zhang. Dual role of PKC in modulating pharmacomechanical coupling in fetal and adult cerebral arteries. Am J Physiol Regulatory Integrative Comp Physiol 279: R1419-R1429, 2000.--This study tested the hypothesis that protein kinase C (PKC) has dual regulation on norepinephrine (NE)-mediated inositol 1,4,5-trisphosphate [Ins (1,4,5)[P.sub.3]] pathway and vasoconstriction in cerebral arteries from near-term fetal (~140 gestational days) and adult sheep. Basal PKC activity values (%membrane bound) in fetal and adult cerebral arteries were 38 [+ or -] 4% and 32 [+ or -] 4%, respectively. In vessels of both age groups, the PKC isoforms [Alpha], [[Beta].sub.I], [[Beta].sub.II], and [Delta] were relatively abundant. In contrast, compared with the adult, cerebral arteries of the fetus had low levels of [PKC-[Epsilon]. In response to [10.sub.-4] M phorbol 12,13-dibutyrate (PDBu; PKC agonist), PKC activity in both fetal and adult cerebral arteries increased 40-50%. After NE stimulation, PKC activation with PDBu exerted negative feedback on Ins(1,4,5)[P.sub.3] and intracellular [Ca.sup.2+] concentration ([[[Ca.sup.2+]].sub.i]) in arteries of both age groups. In turn, PKC inhibition with staurosporine resulted in augmented NE-induced Ins(1,4,5)[P.sub.3] and [[[Ca.sup.2+]].sub.i] responses in adult, but not fetal, cerebral arteries. In adult tissues, PKC stimulation by PDBu increased vascular tone, but not [[Ca.sup.2+]].sub.i]. In contrast, in the fetal artery, PKC stimulation was associated with an increase in both tone and [[[Ca.sup.2+]].sub.i]. In the presence of zero extracellular [[Ca.sup.2+]], these PDBu-induced responses were absent in the fetal vessel, whereas they remained unchanged in the adult. We conclude that, although basal PKC activity was similar in fetal and adult cerebral arteries, PKC's role in NE-mediated pharmacomechanical coupling differed significantly in the two age groups. In both fetal and adult cerebral arteries, PKC modulation of NE-induced signal transduction responses would appear to play a significant role in the regulation of vascular tone. The mechanisms differ in the two age groups, however, and this probably relates, in part, to the relative lack of PKC-[Epsilon] in fetal vessels. cerebrovasculature; smooth muscle; signal transduction; adrenergic; norepinephrine; protein kinase C; protein kinase C isoenzymes; inositol 1,4,5-trisphosphate; intracellular [Ca.sup.2+], [Ca.sup.2+] channels; vascular tone; fetus; development

Cerebral artery sarcoplasmic reticulum [Ca.sup.2+] stores and contractility: changes with development. Am J Physiol Regulatory Integrative Comp Physiol 279: R860-R873, 2000.--To test the hypothesis that sarcoplasmic reticulum (SR) [Ca.sup.2+] stores play a key role in norepinephrine (NE)-induced contraction of fetal and adult cerebral arteries and that [Ca.sup.2+] stores change with development, we performed the following study. In main branch middle cerebral arteries (MCA) from near-term fetal (~140 days) and nonpregnant adult sheep, we measured NE-induced contraction and intracellular [Ca.sup.2+] concentration ([[[Ca.sup.2+]].sub.i]) in the absence and presence of different blockers. In adult MCA, after thapsigargin ([10.sup.-6] M), the NE-induced responses of tension and [[[Ca.sup.2+]].sub.i] were 37 [+ or -]5 and 47 [+ or -] 7%, respectively, of control values (P [is less than] 0.01 for each). In the fetal artery, in contrast, this treatment resulted in no significant changes from control. When this was repeated in the absence of extracellular [[Ca.sup.2+].sub.i], adult MCA increases in tension and [[[Ca.sup.2+]].sub.i] were 32 [+ or -] 5 and 13 [+ or -] 3%, respectively, of control. Fetal cerebral arteries, however, showed essentially no response. Ryanodine (RYN, 3 x [10.sup.-6] to [10.sup.-5] M) resulted in increases in tension and [[[Ca.sup.2+]].sub.i] in both fetal and adult MCA similar to that seen with NE. For both adult and fetal MCA, the increased tension and [[[Ca.sup.2+]].sub.i] responses to RYN were essentially eliminated in the presence of zero extracellular [Ca.sup.2+]. These findings provide evidence that in fetal MCA, in contrast to those in the adult, SR [Ca.sup.2+] stores are of less importance in NE-induced contraction, with such contraction being almost wholly dependent on [Ca.sup.2+] flux via plasma membrane L-type [Ca.sup.2+] channels. In addition, they suggest that in both adult and fetal MCA, the RYN receptor is coupled to the plasma membrane [Ca.sup.2+]-activated [K.sup.+] channel and/or L-type [Ca.sup.2+] channel. cerebrovascular circulation; vascular smooth muscle; sympathetic nervous system; norepinephrine; intracellular calcium; thapsigargin; cyclopiazonic acid; ryanodine; L-type calcium channel; fetus; adult

Angeles, Danilyn M., James Williams, Lubo Zhang, and William J. Pearce. Acute hypoxia modulates 5-HT receptor density and agonist affinity in fetal and adult ovine carotid arteries. Am J Physiol Heart Circ Physiol 279: H502-H510, 2000.--In light of recent observations that receptor-ligand binding and coupling are physiologically regulated, the present study examined the hypothesis that the direct effects of hypoxia on vascular contractility involve modulation of pharmacomechanical coupling via changes in agonist affinity and/or receptor density. Because the direct effects of hypoxia on vascular smooth muscle contractility can vary with age, we carried out these experiments using both fetal and adult arteries. In common carotid arteries from near-term fetal and adult sheep, hypoxia ([PO.sub.2] = 9-12 Torr for 30 min) reduced the maximum responses to potassium by 17.8 [+ or -] 3.5% (fetus) and 20.5 [+ or -] 2.2% (adult), significantly reduced the p[D.sub.2] for 5-HT in the fetus (7.01 [+ or -] 0.1 to 6.3 [+ or -] 0.2) but not the adult (6.1 [+ or -] 0.1 to 6.0 [+ or -] 0.1), and significantly reduced 5-HT-induced maximum contractions (as % maximum response to 120 mM [K.sup.+]) not in the fetus (from 114 [+ or -] 7 to 70 [+ or -] 10%, not significant) but only in the adult (from 83 [+ or -] 15 to 25 [+ or -] 7%, P [is less than] 0.05) arteries. Hypoxia significantly attenuated 5-HT binding affinity (p[K.sub.A], determined by partial irreversible blockade with phenoxybenzamine) in both fetal (from 6.5 [+ or -] 0.2 to 6.0 [+ or -] 0.2) and adult arteries (from 6.2 [+ or -] 0.2 to 5.7 [+ or -] 0.1) and also decreased receptor density (fmol/mg protein, determined by competitive binding with ketanserin and mesulergine) in adult (from 18.3 [+ or -] 1.1 to 10.9 [+ or -] 1.0) but not in fetal (21.0 [+ or -] 1.0 to 23.2 [+ or -] 1.4) arteries. These results suggest that acute hypoxia modulates receptor-ligand binding via age-dependent modulation of agonist affinity and receptor density. These effects may contribute to hypoxic vasodilatation and help explain why the effects of hypoxia on vascular contractility differ between fetuses and adults. ketanserin; competition binding; maturation; mesulergine; ontogeny