モルモット結腸紐平滑筋糸田胞にみられる二種類の非選択性陽イオンチャネル電流

The effects of carbachol (CCh) and caffeine on membrane ionic currents were investigated in smooth muscle cells of the guinea-pig taenia caeci (GPTC) using conventional and nystatin-perforated patch recording configurations. Application of CCh (1-10μM) induced an inward current (ICCh) (-43.9±7.6 pA, n=26) at a holding potential of -50 mV. Atropine (1/μM) blocked this CCh action, indicating that muscarinic receptor activation is involved. The reversal potential (ERev) of ICCh was investigated by applying short ramp pulses at a frequency of 3 Hz throughout the period of CCh application in order to determine whether the current composition changed during the response. ERev of ICCh at several different times was found to converge around +6.7±0.25 mV (n=3), indicating that ionic species were constant and the current showed no significant Cl- current component. On substitution of external Na+ for Tris+, ERev of ICCh shifted in the negative direction by about 23 mV, indicating that nonselective cation channels (NSCC) are mainly responsible for ICCh and that Na+ is the dominant species of the current. Application of caffeine, which mobilizes Ca2+ from an internal store, failed to mimic the action of CCh, indicating that such Ca2+ cannot by itself activate the channels responsible for ICCh. Caffeine instead induced a unitary current activity which was quite differnet to that evoked by CCh. The slope conductance of this current was estimated to be about 430 pS and the reversal potential of the current was around +10 mV, which suggested that this current also flows through another NSCC. In order to establish the mechanism of activation of these NSCCs, the effect of a bath application of Ca2+ inophore A23187 was tested; A23187 failed to induce any current. Internal Ca2+ thus seems to be unable to activate these channels. Our results indicate the presence of two different NSCCs in GPTC. The first can be activated directly via CCh coupling muscarinic receptors but cannot be activated by internal Ca2+ although internal Ca2+ was shown to enhance current. The second is a high conductance channel which can be activated by caffeine although, similarly, it cannot be activated by internal Ca2+. The physiological significance of these two types of NSCCs has been discussed