Potential‐dependent inward currents in single isolated smooth muscle cells of the rat ileum.

Abstract

SUMMARY1. Calcium (ICa) and sodium (INa) 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 INa was blocked, two components of 'Ca' low-voltage activated or ICalow and high-voltage activated or ICahigh' were observed in the newborn rat ileal cells. ICa high and ICalow have differing voltage ranges of activation and steady-state inactivation and time courses of recovery from inactivation. Potential dependence of ICalow was much steeper and shifted toward negative membrane potential than that for ICahigh (slope factors and the potential of halfmaximal inactivation were 13-6 and -60X6 and 8X8 and -49 mV for ICalow and ICahigh' correspondingly).3. Nifedipine at the high concentration of 30 gim exerted no effect on ICa low and only slightly suppressed ICa high' decreasing its peak to 0'81 +004 (n= 7) at the holding potential of -80 mV and to 0-66 + 0.05 (n = 3) at -50 mV. ICahigh was suppressed significantly by Cd2+ ions, while Ica,low was more sensitive to Ni2+ ions. 7. INa 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 INa, which was completed in about 150 ms, was found.8. Steady-state inactivation ofINa 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 Ms 9500 S. V. SMIRNOV, A. V. ZHOLOS AND M. F. SHUBA INa from inactivation was monoexponential and slowed with depolarization, from 1841 + 1P4 ms (n = 5) at the holding potential of -80 mV to 46f5 + 3.0 ms (n = 5) at -60 mV.9. In the presence of 2-5 mM-Ca2+ the dependence of INa 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.

Regenerative potentials evoked in circular smooth muscle of the antral region of guinea‐pig stomach

“…However, such channels have not been described in this tissue (Noack et al 1992). Moreover, if this were the case, the response should occur after a short latency; intestinal Ca¥ channels activate rapidly (Smirov et al 1992) and the regenerative release of Ca¥ from an internal store occurs abruptly (Schiefer et al 1995;Imaizumi et al 1996). Voltage-sensitive release of Ca¥ from intracellular stores has not been demonstrated in smooth muscle but has been in cardiac muscle (Howlett & Ferrier, 1997).…”

Molecular Physiology of Low-Voltage-Activated T-type Calcium Channels

“…Isradipine (10 nM) inhibited the plateau phase with no effect on the rising phase, suggesting that T-type channels might play a pacemaker role in these cells. LVA currents have been described in SMCs from bronchi (185,448), ileum (373), colon (445), bladder (382), and uterus (453). The results with pregnant human uterine SMCs are notable because the T-type currents (Ϫ3 pA/pF in 1.8 mM Ca 2ϩ ) were larger than the L-type currents (453).…”

T-type Ca2+ channels in non-vascular smooth muscles

“…It was virtually absent in neonatal cells, but increased during development, until in aged rats the proportion was about one-third of the net current density, especially as the L-type component also declined in later age. Of interest is an earlier observation in rat ileal myocytes, where two components of inward current were also measured [36] channels, as they are unaffected by dihydropyridines [39]. Such a current has been identified in ICC from dog and mouse G-I tract [40,41].…”