Calcium channel currents in Xenopus oocytes injected with rat skeletal muscle RNA.

N. Dascal, I. Lotan, E. Karni, A. Gigi

Research output: Contribution to journalArticlepeer-review

Abstract

1. Ba2+ currents (IBa) 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 IBa between ‐5 and ‐20 nA at 0 mV. However, in ‘variant’ native oocytes of four frogs, IBa exceeded ‐30 nA and reached up to ‐100 nA. In oocytes injected with SkM RNA, IBa of up to ‐250 nA was observed. 3. In SkM RNA‐injected oocytes and ‘variant’ native oocytes, the decay of IBa displayed two kinetic components. The faster component was selectively blocked by 40‐100 microM‐Ni2+ and thus was termed the Ni(2+)‐sensitive IBa. The slower component was Ni2+ resistant, being inhibited only 10‐20% by 100‐200 microM‐Ni2+. The half‐activation and the half‐inactivation voltages of the Ni(2+)‐sensitive IBa were more negative (by 14.5 and 28.7 mV, respectively) than those of the Ni(2+)‐resistant IBa. 4. Neither Ni(2+)‐sensitive nor Ni(2+)‐resistant IBa in native or SkM RNA‐injected oocytes were affected by dihydropyridine antagonists nifedipine and (+) PN 200‐110 (1‐10 microM), by the dihydropyridine agonist (‐)Bay K 8644 (0.01‐2 microM), or by verapamil below 50 microM. IBa was blocked by diltiazem (half‐block at about 500 microM). Thus, the pharmacology of IBa 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 Ni(2+)‐sensitive or Ni(2+)‐resistant IBa 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.

Original languageEnglish
Pages (from-to)469-490
Number of pages22
JournalJournal of Physiology
Volume450
Issue number1
DOIs
StatePublished - 1 May 1992

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