The effects of neurokinin A (NKA) on freshly isolated smooth muscle cells from the rat vas deferens were investigated electrophysiologically. NKA (10-7 M) prolonged the duration of action potentials without affecting resting potentials. Under voltage-clamp conditions, NKA (10-7 M) suppressed an outward K+ current activated by depolarizing voltage steps. Substance P (SP, 10-7 M) did not affect the K+ current, suggesting that NKA is more potent in suppressing the K+ current than SP is, as is also true for the contractions of smooth muscles of the rat vas deferens. The suppression of the K+ conductance now reported may contribute to NKA-induced smooth muscle contraction.
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