Difference between adrenergic beta 1- and beta 2-blocking effects on isoproterenol-induced Ca spike suppression in guinea-pig taenia coli.

Ca spike suppressions induced by isoproterenol (IsP) and a beta 2-agonist, 5-hydroxymethyl-6-hydroxy-2-isopropylamino-1,2,3,4-tetrahydronaphthalene -1-ol (AA497), were investigated in the presence of butoxamine or practolol. The relaxations were isotonically recorded, and the Ca spike frequency was recorded using the single sucrose gap method. IsP-induced relaxation was not inhibited by butoxamine (Butox, 0.16 microM), but was inhibited by practolol (Prac, 0.188 microM). In 24 mM K+-Krebs' solution, AA497 caused relaxation in a lower dose range and suppressed to a small extent the Ca spike frequency in a higher dose range, as was observed for IsP-induced curves of log dose-spike frequency and log dose-relaxation. In normal K+-Krebs' solution, both IsP and AA497 greatly suppressed the Ca spike frequency. IsP (1.21 microM)-induced suppression of the Ca spike frequency was blocked by Prac (113 microM), and it was blocked by Butox (96 microM) to a greater extent. AA497-induced suppression of the spikes was not blocked by Prac (37.6 microM), but completely blocked by Butox (32 microM). These selective inhibitory effects of butoxamine on AA497-induced and IsP-induced Ca spike suppression demonstrate that in the adrenergic beta-receptor-mediated process in taenia coli, beta 2-mechanisms are more closely related to the Ca spike suppression than the beta 1-mechanisms are.