Abstract

The mechanism of a synthetic atrial peptide (APII)-induced inhibition of smooth muscle contractility was investigated by studying its effects on tension development and 45Ca fluxes in isolated rabbit aorta. APII (10(-9) to 10(-7) M) produced a dose-dependent relaxation of contractions produced by alpha adrenoceptor activation with norepinephrine (NE; 10(-6) M). APII was a potent relaxant of NE contraction with an IC50 = 1.1 X 10(-8) M, with 10(-7) M APII causing a 97% relaxation. APII also produced a dose-dependent inhibition of NE contraction when added to the resting muscle before the exposure to NE. The relaxing effects of APII were found to be endothelium independent. In contrast, APII was only marginally effective in relaxing high-K+ contraction, with 10(-7) M APII causing only 17% relaxation. Furthermore, when a NE contraction was obtained on top of a high-K+ contraction, APII was still capable of relaxing the NE component. APII was similarly more effective in inhibiting NE-stimulated 45Ca influx than high-K+-stimulated 45Ca influx, indicating selective action of APII on the receptor-operated Ca++ channels. This was in contrast to D600, a well known Ca++ antagonist, which had a more selective inhibitory effect on the potential-operated Ca++ channels. The data presented indicate that APII is a potent relaxant of contractions produced by receptor-agonists involving 45Ca influx through receptor-operated Ca++ channels. APII may also prove to be a very useful tool to further distinguish and define receptor-operated Ca++ channels and potential-operated Ca++ channels in vascular smooth muscle.