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(-5)M), an alphareceptor blocking agent. This action of isoprenaline showed dose-dependency (ED 50: 10(-5)M) and was abolished completely by beta-receptor blocking agent (propranolol, 10(-5)M). The effect of isoprenaline (10(-5)M) 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+]o-free solution or ouabain (10(-5)M) enhanced the effect of isoprenaline. The effect of isoprenaline is inhibited by ouabain (10(-5)M) 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.
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