Effect of apamin on responses to BRL 34915, nicorandil and other relaxants in the guinea‐pig taenia caeci

Abstract

1 BRL 34915 (4–64 × 10⁻⁷ M), isoprenaline (0.5–32 × 10⁻⁸ M) and nicorandil (4–64 × 10⁻⁶ M) produced a slowly-developing relaxation of spontaneous tone of the guinea-pig taenia caeci; with no after-contraction on washout. These inhibitory responses were unaffected by apamin (10⁻⁷ M). 2 Adenosine triphosphate (0.06-2 × 10⁻³ M) and noradrenaline (1–16 × 10⁻⁷ M) produced a rapid inhibition of spontaneous tone with a prominent after-contraction, especially on washout. Both the inhibitory effect and the rebound contraction were abolished by apamin (10⁻⁷ M). 3 Exposure to both BRL 34915 (64 × 10⁻⁷ M) and to nicorandil (64 × 10⁻⁶ M) produced an increase in the ⁸⁶Rb efflux rate coefficient which was unaffected by apamin (10⁻⁷ M). 4 Exposure to isoprenaline (32 × 10⁻⁸ M) had no effect on the ⁸⁶Rb efflux rate coefficient. 5 Exposure to noradrenaline (16 × 10⁻⁷ M) produced an increase in the ⁸⁶Rb efflux rate coefficient which was abolished by apamin (10⁻⁷ M). 6 The results confirm that both BRL 34915 and nicorandil are capable of opening potassium channels in smooth muscle but show that the channel is not apamin-sensitive.