POTENTIAL-DEPENDENT INWARD CURRENTS IN SINGLE ISOLATED SMOOTH-MUSCLE CELLS OF THE RAT ILEUM

1. Calcium (I(Ca)) and sodium (I(Na)) currents were studied in single smooth muscle cells freshly isolated from both the newborn (1-3 days old) and adult rat ileum, using the patch-clamp technique (whole-cell configuration). 2. Under conditions when I(Na) was blocked. two components of I(Ca), low-voltage activated or I(Ca,low) and high-voltage activated or I(Ca,high), were observed in the newborn rat ileal cells. I(Ca,high) and I(Ca,low) have differing voltage ranges of activation and steady-state inactivation and time courses of recovery from inactivation. Potential dependence of I(Ca,low) was much steeper and shifted toward negative membrane potential than that for I(Ca,high) (slope factors and the potential of half-maximal inactivation were 13.6 and - 60.6 and 8.8 and - 49 mV for I(Ca,low) and I(Ca,high), correspondingly). 3. Nifedipine at the high concentration of 30-mu-m exerted no effect on I(Ca,low) and only slightly suppressed I(Ca,high), decreasing its peak to 0.81 +/- 0.04 (n = 7) at the holding potential of - 80 mV and to 0.66 +/- 0.05 (n = 3) at - 50 mV. I(Ca,high) was suppressed significantly by Cd2+ ions, while I(Ca,low) was more sensitive to Ni2+ ions. 4. Results presented here suggest that the properties of high-voltage-activated (HVA) Ca2+ channels in the rat small intestine are quite different to those described for L-type Ca2+ channels found in other smooth muscles. It is proposed that HVA Ca2+ channels are similar to N-type Ca2+ channels. 5. Comparison of Ca2+ currents in newborn and adult rat ileal cells showed that the contribution of I(Ca,low), to the net Ca2+ current was negligible in adults, whereas the properties of HVA Ca2+ channels were similar in the neonatal and adult animals. 6. I(Na), studied in nominally Ca2+-free physiological salt solution, activated in the voltage range between - 50 and - 40 mV and reached its peak at - 10 mV. I(Na) was blocked in a dose-dependent manner by TTX with an apparent dissociation constant of 4.5 nM. 7. I(Na) decay was monoexponential in the voltage range studied and its time constant decreased monotonically with membrane depolarization from 4.7 +/- 0.2 ms (n = 6) at - 30 mV to 0.51 +/- 0.03 ms (n = 7) at 20 mV. Also, a process of slow inactivation of I(Na), which was completed in about 150 ms, was found. 8. Steady-state inactivation of I(Na) was described by the Boltzmann equation with a half-inactivation potential of - 60.3 mV and a slope factor of 9.8 mV. Recovery of I(Na) from inactivation was monoexponential and slowed with depolarization, from 18.1 +/- 1.4 ms (n = 5) at the holding potential of - 80 mV to 46.5 +/- 3.0 ms (n = 5) at - 60 mV. 9. In the presence of 2.5 mM-Ca2_ the dependence of I(Na) inactivation on the membrane potential was shifted by about 13 mV toward positive voltages, i.e. more than 50 % of Na+ channels are able to be activated in the voltage range between - 60 and - 50 mV which is close to the resting potential in many intestinal smooth muscle at the physiological Ca2+ concentration.