Activations of the Ca dependent K channel by Ca released from the sarcoplasmic reticulum of mammalian smooth muscles

In mammalians mooth muscles, the outward K current recorded using the whole cell voltage clamp or patch clamp methods can be classified into the Ca-dependent and - independent K currents. The former is sub-classified into the extra- and intra-cellular Ca dependent K current. The intra-cellular Ca dependent K current has a close relation to Ca released from the sarcoplasmic reticulum, i.e. Ca released by inositol 1,4,5-triphosphate (InsP₃), ryanodine or Ca ionophores (A23187 or ionomycin) modify the appearance of the K current. The transient (Ca dependent) outward current evoked by depolarization pulses, as measured using the whole cell voltage clamp method, is closely related with after-hyperpolarization of the action potential as recorded using the microelectrode method and is postulated to be due to activations of the Ca-induced Ca release mechanism in the sarcoplasmic reticulum. The oscillatory (Ca dependent) outward K current is closely related with the amount of Ca released from the sarcoplasmic reticulum during the long depolarization induced by electrical stimulation (command pulse) or applications of Ca releasers such as InsP₃ or ryanodine. In this review, the Ca dependent K current recorded from smooth muscle cells is compared with the influx and release of Ca.