Studies on alpha adrenoceptors in guinea-pig peripheral lung strips

Contractile responses of guinea-pig peripheral lung strips to noradrenaline were determined in the presence of propranolol (2.5 X 10(-6) mol/l). All strips (n = 44) contracted to noradrenaline and a geometric mean EC50 of 1.4 X 10(-6) mol/l (1.1 X 10(-6) mol/l, 1.8 X 10(-6) mol/l 95% confidence limits) was obtained. Contractions were antagonised by phentolamine (5 X 10(-7)-10(-5) mol/l) and by prazosin (10(-8)-10(-7) mol/l). Dose-ratios (DR) were calculated and log (DR-1) was plotted against log concentration of antagonist to yield slopes (+/- SE) of 0.84 +/- 0.14 and 0.73 +/- 0.22 respectively which were not significantly different from unity. A pA2 value (+/- SE) of 6.7 +/- 0.2 was obtained for phentolamine and 7.5 +/- 0.1 for prazosin. Yohimbine (10(-7)-10(-5) mol/l) did not significantly affect the maximal tension generated or the EC50 values for noradrenaline. These results suggest that alpha 1 adrenoceptors are mediating the contractile responses to noradrenaline. In the presence of cocaine (10(-5) mol/l, n = 18), normetanephrine (2 X 10(-5) mol/l, n = 15), hydrocortisone (2.5 X 10(-5) mol/l, n = 15) and normetanephrine (2 X 10(-5) mol/l) plus cocaine (10(-5) mol/l, n = 15) pA2 values for phentolamine of 6.9, 6.7, 6.6, and 6.3 respectively were obtained. Since these pA2 values are not significantly different from 6.7, it is unlikely that this original pA2 value, which is lower than values obtained with phentolamine at alpha-adrenoceptors in other tissues, may be explained by neuronal or extraneuronal uptake of noradrenaline.