Contractile responses to epinephrine, serotonin, and potassium in arterial smooth muscle

Effects of epinephrine, serotonin, and KCl on development of isometric tension were studied in strips of rat femoral and tail artery smooth muscle. Strkining differences were detected between the preparations. Tension development in tail artery smooth muscle was greatest with epinephrine, intermediate with serotonin, and least with K+. In femoral preparations tension developed was greatest with serotonin. Smaller but comparable contractions were elicited with epinephrine and K+. Theoretical and experimental dose-response curves for epinephrine and serotonin agreed closely, whereas curves for K+ differed markedly. The relationship between fractional response and fractional stimulus was hyperbolic for epinephrine and serotonin but sigmoid for K+. The Hill coefficient for serotonin was about 1, slightly smaller than 1 for epinephrine (0.5 to 0.8) and significantly greater for K+ (5 to 5). These findings show that responses elicited with either epinephrine or serotonin are mechanistically consistent with receptor occupancy theory. They suggest that specific receptors for K+ exist and that they may involve positive cooperative interactions similar those described for receptor mechanisms in electroplax.