Calcium current in embryonic Xenopus muscle cells in culture

We have investigated the appearance of calcium current in Xenopus muscle cells in 1- to 6-day-old cultures. Whole cell currents were recorded using a patch-clamp amplifier with sodium and potassium replaced with tetraethylammonium and cesium, respectively, and BaCl2 used in place of CaCl2. When the muscle membrane was depolarized above -30 mV, a slow inward current was activated, the current reached a peak amplitude near 0 mV, and an outward current became apparent above +10 mV. This slow current was enhanced by adding barium or Bay K 8644 to the extracellular recording solution and was blocked by the addition of cobalt, cadmium, or the dihydropyridines nifedipine or (+)PN 200-110. Taken together these results indicate the presence of an inward calcium current mediated through L-type channels. Thirty-one percent of the cells examined on the first day in culture showed no discernible slow inward current; however, as the age of the culture increased, all cells showed slow inward current and there was an increase in the amplitude of the current. A small proportion of the muscle cells (5 out of 34) also showed a fast activating and inactivating inward current. This current, which activated at more hyperpolarized potentials (-40 mV) was only present when 5 mM ATP was included in the internal recording solution. It also appeared to be mediated through a calcium channel but not a dihydropyridine, sensitive channel.