Abstract
The relaxing effect of isoprenaline on [Na+] o-free induced contracture of the guinea-pig taenia coli was investigated in presence of phentolamine (10-5M), an alphareceptor blocking agent. This action of isoprenaline showed dose-dependency (ED 50: 10-5M) and was abolished completely by β-receptor blocking agent (propranolol, 10-5M). The effect of isoprenaline (10-5M) disappeared gradually during long exposure to [Na+] o-free solution and when short application of sodium ions to the perfusing solution, the effect of isoprenaline was restored. Pretreatment of [K+] ofree solution or ouabain (10-5M) enhanced the effect of isoprenaline . The effect ofisoprenaline is inhibited by ouabain (10-5M) by about 50%. These observations strongly suggest that the inhibitory effect of isoprenaline may be mainly mediated through activation of Na-pump.However, direct activation of the drug on Na+-Ca2+ exchange, Ca-pump, or Ca-uptake into cellular Ca store sites can not be ruled out.
