The effects of bradykinin and the bradykinin potentiating peptide BPP5a on the electrical and mechanical responses of the guinea-pig taenia coli.

1 By means of the double sucrose-gap technique, the effects of bradykinin and the bradykinin potentiating peptide BPP(5a) were compared on the guinea-pig taenia coli under a number of experimental conditions.2 In normal Krebs solution the response to bradykinin was mostly a slight stimulation, characterized by a depolarization, an increase in spontaneous spike activity and a contraction. If BPP(5a) caused any effect at all, it was stimulation of the spike activity but without depolarization. Since the effect of bradykinin was little affected by an increase in dose, a potentiating effect of BPP(5a) could not be determined.3 Spontaneous spikes with a 5 to 7 s rhythm and prepotentials at their base were inhibited by bradykinin, whereas they were stimulated by BPP(5a).4 Oscillatory potentials (slow waves) induced by a calcium and magnesium-free medium were also suppressed by bradykinin and stimulated by BPP(5a). This effect of bradykinin was accompanied by a depolarization and a decrease in membrane resistance, phenomena not found after administration of BPP(5a).5 The amplitude of spontaneous spikes induced by potassium-depolarization was suppressed by bradykinin, even though the membrane resistance and potential had been decreased. BPP(5a) produced either no effect or a small stimulatory effect without influencing the membrane resistance.6 Reduction of the calcium concentration to 0.25 mM enhanced the stimulatory responses to both bradykinin and BPP(5a), especially the spike activity and depolarization. In this case the membrane resistance was increased by bradykinin as well as BPP(5a). These effects, especially those of BPP(5a), were inhibited by reduction of the sodium concentration to 15.5 mM. Reduction of the chloride concentration to 9.7 mM decreased rather than increased the stimulatory effects of both bradykinin and BPP(5a). Under these conditions bradykinin did not decrease the membrane resistance.7 Bradykinin can have both inhibitory and stimulatory effects on the taenia coli whereas BPP(5a) has only a stimulatory effect. Since under certain conditions both responses to bradykinin are accompanied by a sodium-dependent depolarization and decrease in membrane resistance, not influenced by lanthanum to any extent, it is suggested that bradykinin induces an increase in sodium conductance of the membrane. Under all the conditions investigated, except in low calcium, BPP(5a) did not affect the membrane potential and resistance. Thus, the underlying cause of its stimulatory effect is probably different from that of bradykinin.