Potassium induces relaxation and hyperpolarization of circular muscles but contraction of longitudinal muscles of pig duodenum.

The mechanisms by which K+ relaxes circular muscles of pig duodenum were investigated, and compared with the response of the longitudinal muscles to K+. Circular muscles were concentration-dependently relaxed by 8.3-23.6 mM K+, but contracted by 47.2-143.4 mM K+. Longitudinal muscles were contracted by 11.8-94.4 mM K+. The relaxation of circular muscles was correlated with hyperpolarization (4 mV), but evoked Ca2+ spikes were not suppressed. Neither ouabain (0.14 microM) nor phentolamine (10 microM) blocked the relaxation, but tetrodotoxin (TTX, 0.63 microM) blocked both the relaxation and hyperpolarization. Mesaconitine (0.16 microM) increased the relaxation. Inhibitory junction potentials and concomitant relaxations were also blocked by TTX. The results suggest that K+-induced relaxation is caused by the release of a non-adrenergic inhibitory transmitter.