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We describe the Ca2+movements in smooth muscle cells at rest and during activation and relaxation as deduced from transplasmalemmal Ca2+fluxes and contractile responses. The general picture which emerges is: the resting cell has a [Ca2+]cytbelow 10−7M and large gradients are poised across both the cell membrane and intracellular membranes. Excitation opens up Ca2+channels which are linked to receptors and, if depolarization occurs, to other channels not linked to specific receptors but capable of sensing the membrane potential. Receptor activation also leads to release of Ca2+from a limited intracellular Ca2+pool which is superficially located because it has to be refilled from the outside. Relaxation is effected by Ca2+accumulation by another intracellular Ca2+pool, very likely sarcoplasmic reticulum, which does not release Ca2+during activation. The sarcoplasmic reticulum Ca2+pump can also decrease initial activation of the myofilaments. Elevation of cAMP levels may inhibit contraction by stimulating the sarcoplasmic reticulum Ca-ATPase. An enormous amount of research is still required to prove the above scheme and to localize and quantitate the various intracellular Ca2+pools.

The effects of a stable analogue of PGH2 on 45Ca fluxes and tension were studied in the isolated rabbit aorta. U-44069 produced maximal tension responses at a concentration of 10(-7) M. The analogue increased smooth muscle Ca2+ content and increased the unidirectional 45Ca influx. U-44069 also induced contractions of reduced magnitude in the absence of external Ca2+ and caused a transient stimulation of 45Ca efflux, suggesting that the analogue releases intracellularly bound Ca2+. Both the contractile response in CA2+-free media and the stimulation of 45Ca efflux were greatly attenuated by prior exposure to norepinephrine. The analogue-induced contractions in Ca2+-free media were also prevented by prior exposure to histamine but not angiotensin II. D-600 completely blocked the U-44069-induced gain in calcium content (IC50 = 10(-6) M) but inhibited the contractile response by only 40%. It was concluded that the PGH2 analogue activates rabbit aorta by activating a CA2+ influx pathway and releasing a fraction of the intracellular Ca2+ pool that is sensitive to norepinephrine and other agonists.

The intrarenal sites of action of atrial natriuretic peptide (ANP) have not been resolved fully. Although ANP relaxes in vitro preparations of smooth muscle and exerts profound effects on renal hemodynamics, the effects of ANP on resistance vessels remain a subject of controversy. In the present study, we utilized an in vitro perfused hydronephrotic rat kidney model to assess directly the actions of ANP on renal microvessels during norepinephrine (NE)-induced renal vasoconstriction. Perfusion pressure was maintained constant, and perfusate flow to the kidney was monitored while the afferent arteriole (AA) and efferent arteriole (EA) were visualized using videomicroscopy. Renal AA and EA diameters were measured by computer-assisted image analysis. NE (0.3 microM) decreased renal perfusate flow from 11.8 +/- 1.7 (S.E.) to 5.8 +/- 1.2 ml/min (P less than .005), decreased AA diameter from 20.2 +/- 0.7 to 14.3 +/- 0.8 mu (P less than .005) and decreased EA diameter from 18.6 +/- 1.2 to 15.1 +/- 1.2 mu (P less than .005). ANP (human ANF-(4-28), anaritide, Wyeth) completely reversed the NE-induced AA vasoconstriction with an IC50 of 9.3 +/- 5.6 nM. In contrast, ANP caused a further decrease in EA diameter at concentrations up to 10(-8) M, and elicited a slight dilation of the EA at 10(-7) M. In normal rat kidneys perfused under identical conditions, NE decreased renal perfusate flow, glomerular filtration rate and filtration fraction. ANP increased glomerular filtration rate and filtration fraction above control levels, with maximal effects at 10(-8) M.(ABSTRACT TRUNCATED AT 250 WORDS)

We have demonstrated previously that the organic Ca++ antagonist diltiazem augments the glomerular filtration rate (GFR) of the isolated perfused rat kidney during norepinephrine (NE) - induced vasoconstriction. These earlier studies, however, did not elucidate the precise mechanism or site of action responsible for this effect. Nisoldipine (NIS) interacts with the same Ca+2 channels as diltiazem but differs in its physicochemical properties, binding characteristics and tissue specificity. We examined, therefore, the effects of NIS using an identical model. Renal perfusate flow and GRF were assessed in the isolated perfused rat kidney under conditions of constant renal perfusion pressure (100 mm Hg). NIS (10(-7) M) completely reversed the NE-induced reduction in GFR but was significantly less effective in augmenting renal perfusate flow. In additional series of experiments, filtration pressure was estimated during these manipulations by monitoring ureteral pressure during ureteral occlusion (stop-flow pressure). The NE-induced decrease in GFR was accompanied by a reduction in stop-flow pressure, which was abolished by the subsequent administration of NIS. Thus, nisoldipine preferentially attenuated NE-induced constriction of preglomerular resistance vessels but was less effective in reversing the effects of NE on postglomerular arterioles. These findings indicate that separate postreceptor mechanisms mediate the activation of pre- and postglomerular vessels by NE.