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The ability of basal release of endothelium derived relaxing factor (EDRF) to alter contractile events in phenylephrine (PE)-triggered contraction was tested on ring segments of the thoracic aorta removed from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). In normal medium, PE (1 microM) elicited similar whole contractions in endothelium denuded arteries of SHR and WKY. The presence of endothelium only reduced the WKY response. On aorta incubated in a Ca2+ free-medium, PE (1 microM) induced an initial phasic contraction due to intracellular Ca2+ release. This was followed by a tonic contraction after Ca2+ (2.5 mM) was restored to the bath. This sustained contraction was dependent on extracellular calcium influx. Identical phasic and tonic contractions were observed in endothelium denuded rings of SHR and WKY. However, the presence of endothelium only reduced the sustained contraction of WKY arteries. When experiments were carried out in medium containing D600 (1 microM), the presence of endothelium diminished the whole contraction of both SHR and WKY rings whereas the sustained but not the phasic contractions of WKY was also inhibited. This inhibitory effect of endothelium on WKY sustained contraction was significantly higher in the presence of D600. The calcium antagonist reduced both the whole and the tonic contractions of all preparations but was ineffective on the phasic one. The D600 inhibitory action on the sustained contraction was more pronounced in denuded SHR rings than in the corresponding WKY arteries. Thus it is concluded that there is a basal influence of endothelium in both SHR and WKY. Under our conditions, the endothelial function inhibited the extracellular Ca2+ influx and especially the part of Ca2+ influx insensitive to D600. This part of Ca2+ influx is diminished in SHR and thus the efficacy of endothelium products (e.g. EDRF) is reduced in this strain.

The effects of atrial natriuretic factor (ANF) on contractile events triggered by phenylephrine (PE) were investigated in rat aorta. Isometric contraction of endothelium-free rat aorta rings was recorded in Ca2+-free medium containing 1 mM EGTA. PE (1 microM) induced a phasic contraction and a sustained contraction following addition of Ca2+ (2.5 mM) to the medium. The phasic contraction was due to intracellular Ca2+ release whereas the sustained one was dependent on extracellular Ca2+ influx. ANF (1-3 nM) and prazosin (3-10 nM) both reduced the two types of contraction. However, during the sustained contraction, a rhythmic activity was observed when ANF, but not prazosin, was used as an inhibitory agent. The calcium antagonist diltiazem (0.1-1 microM) abolished this rhythmic activity, which was attributed to a Ca2+ influx through potential-dependent channels. The results indicate that ANF, unlike prazosin, may discriminate between PE-triggered contractile events in the rat aorta.

The interaction between cicletanine (CIC) and atrial natriuretic factor (ANF) on vessels was studied using the isolated rat aorta as model. Contractions induced by endothelin (ET1), a vasoconstrictor peptide from vascular endothelial cells, and by phenylephrine (PE), an alpha 1-adrenoceptor agonist, were recorded isometrically on aortic rings the endothelium of which had deliberately been damaged. Both agonists produced a concentration-dependent contraction (ET1:EC50 1.8 nM, Emax 2.2 g; PE:EC50 30 nM, Emax 2.4 g) which was antagonized by a 30 min pretreatment with either CIC (100 microM) or ANF (1 nM). However, PE was more sensitive than ET1 to the inhibitory action of CIC or ANF. Moreover, during inhibition by ANF spasmodic contractions were observed with PE but not with ET1; this was not observed when the antagonist was CIC. The inhibitory effect produced by the combination of CIC and ANF was additive with ET1 whereas a potentiation was observed with PE. Thus, on the isolated rat aorta model the contraction induced by PE was more sensitive to the action of ANF than that induced by ET1, which indicates that the two antagonists have a different mechanism of action. A direct action of CIC on the alpha-adrenoceptor might account for the potentiation observed between ANF and CIC against PE.

1. The ability of atrial natriuretic factor (ANF) to alter the different Ca++ events in phenylephrine (PE)-triggered contraction was tested on isolated rat aorta. Isometric contraction of endothelium-free rat aortic ring was recorded in Ca++ free medium containing 1 mM EGTA. 2. PE (1 microM) induced a phasic contraction and a sustained contraction following addition of Ca++ (2.5 mM) to the medium. The phasic contraction is due to intracellular Ca++ release whereas the sustained one is dependent on extracellular Ca++ influx. The Ca++ antagonist D600 (1 microM) reduced by 39% the sustained contraction without affecting the phasic one. 3. ANF (0.3-3 nM) reduced both contractions, but spasmodic contractions were observed during the sustained phase. A similar effect was noted with sodium nitroprusside (NaNP; 10-100 nM) and forskolin (1 microM) but not with prazosin (1 nM). 4. When D600 was added to the medium either before or after the initial contraction, the spasmodic contractions were completely abolished. Nifedipine (0.1 microM) had a similar effect when added during the sustained phase, while in contrast, addition of the Ca++ agonist BAY K8644 (0.1 microM), increased the frequency of the spasmodic contractions. 5. It can be concluded that in rat aorta ring, PE induces Ca++ release and Ca++ influx via receptor linked channels and potential dependent channels. ANF, NaNP but also forskolin inhibit both Ca++ release and Ca++ influx via receptor linked channels, while unlike prazosin, these agents are less effective in preventing Ca++ influx via potential dependent channel.