Effect of dopamine on the Ca²⁺-dependent K⁺ currents in isolated single gastric myocytes of the guinea-pig

We have reported that dopamine potentiates spontaneous contractions dose-dependently in guinea-pig antral circular muscle strips (Hwang et al, 1991). To clarify the underlying excitatory mechanism of dopamine on the gastric smooth muscle, the effects of dopamine on voltage-dependent Ca² currents and Ca²⁺ dependent K⁺ currents were observed in enzymatically dispersed guinea-pig gastric myocytes using the whole-cell voltage-clamp technique. Experiments were also done using isometric tension recording and conventional intracellular microelectrode techniques. 1) The effect of dopamine on the spontaneous contraction of antral circular muscle strips of the guinea-pig was excitatory in a dose-dependent manner, and was blocked by phentolamine an α-adrenoceptor blocker. 2) The slow waves were not changed by dopamine. 3) The voltage-operated inward Ca²⁺ current was not influenced by dopamine. 4) The Ca²⁺-dependent K⁺ outward current, which might reflect the changes of intracellular calcium concentration, was enhanced by dopamine. This effect was abolished by phentolamine. 5) The enhancing effect of dopamine on the Ca²⁺-dependent K⁺ current disappeared with heparin which is known to block the action of InsP₃. From these results, it is suggested that dopamine acts via InsP₃-mediated Ca²⁺ mobilization from intracellular stores and such action potentiates the spontaneous contraction of guinea-pig gastric smooth muscle.