CALCIUM-CHANNEL CURRENTS IN XENOPUS OOCYTES INJECTED WITH RAT SKELETAL-MUSCLE RNA

1. Ba2+ currents (I(Ba)) through voltage-dependent Ca2+ channels were studied in Xenopus laevis oocytes injected with heterologous RNA extracted from skeletal muscle (SkM) of young rats, using the two-electrode voltage clamp technique. 2. With 40 or 50 mM-extracellular Ba2+, native oocytes of most frogs displayed I(Ba) between -5 and -20 nA at 0 mV. However, in 'variant' native oocytes of four frogs, I(Ba) exceeded -30 nA and reached up to -100 nA. In oocytes injected with SkM RNA, I(Ba) of up to -250 nA was observed. 3. In SkM RNA-injected oocytes and 'variant' native oocytes, the decay of I(Ba) displayed two kinetic components. The faster component was selectively blocked by 40-100-mu-M-Ni2+ and thus was termed the Ni2+-sensitive I(Ba). The slower component was Ni2+ resistant, being inhibited only 10-20% by 100-200-mu-M-Ni2+. The half-activation and the half-inactivation voltages of the Ni2+-sensitive I(Ba) were more negative (by 14.5 and 28.7 mV, respectively) than those of the Ni2+-resistant I(Ba). 4. Neither Ni2+-sensitive nor Ni2+-resistant I(Ba) in native or SkM RNA-injected oocytes were affected by dihydropyridine antagonists nifedipine and (+) PN 200-110 (1-10-mu-M), by the dihydropyridine agonist (-)Bay K 8644 (0.01-2-mu-M), or by verapamil below 50-mu-M. I(Ba) was blocked by diltiazem (half-block at about 500-mu-M). Thus, the pharmacology of I(Ba) in SkM RNA-injected and in native oocytes was not characteristic of the L-type Ca2+ channel abundant in the skeletal muscle. 5. Destruction of the RNA coding for the channel-forming alpha(1)-subunit of the SkM L-type Ca2+ channel using a hybrid arrest method failed to selectively suppress the appearance of either Ni2+-sensitive or Ni2+-resistant I(Ba) in SkM RNA-injected oocytes. 6. Our results suggest that the appearance of large voltage-dependent Ba2+ currents in SkM RNA-injected oocytes is not due to the expression of the alpha(1)-subunit of the SkM L-type Ca2+ channel. The possibility that the expression of a channel-forming subunit of another Ca2+ channel type underlies one of these currents cannot be rejected. However, since the Ba2+ currents in SkM RNA-injected oocytes resemble those observed in native oocytes, we suggest that their appearance may be the result of an enhanced activity of the native Ca2+ channels, possibly due to the expression of the 'auxiliary' subunits of the SkM Ca2+ channel that form complexes with a native alpha(1)-subunit.