Role of nonselective cation current in muscarinic responses of canine colonic muscle.

The mechanism of muscarinic excitation was studied in colonic muscle strips and isolated cells. In whole cell voltage-clamp studies performed at 33 degrees C utilizing the permeabilized patch technique, acetylcholine (ACh) reduced an L-type Ca2+ current. With K+ currents blocked, depolarization to positive potentials in the presence of ACh elicited outward current. Difference currents showed that ACh activated a voltage-dependent current that reversed at about -8 mV; this current (IACh) had properties similar to the nonselective cation conductance found in other smooth muscle cells. The reversal potential of IACh shifted toward negative potentials when external Na+ was reduced, and the inward current elicited at -70 mV decreased when external Na+ was reduced. IACh was facilitated by internal Ca2+. After the current was activated at a holding potential of -70 mV, depolarizations to -30 to 0 mV elicited influx of Ca2+ via voltage-dependent Ca2+ channels. After repolarization to the holding potential, a large inward tail current was observed. IACh was blocked by Ni2+ and Cd2+ at concentrations of 100 microM or less. Quinine (0.5 mM) also blocked IACh. With the use of the sensitivity of IACh to reduced external Na+ and divalent cations, the role of IACh in responses of intact muscles to ACh was examined. When external Na+ was reduced, ACh failed to increase slow-wave duration, and Ni2+ (50 microM) reversed the depolarization caused by ACh. These data suggest an important role for IACh in the electrical responses of colonic muscles. The contribution of IACh appears to prolong slow waves, which would allow greater entry of Ca2+ and increased force development.