BIOPHYSICAL, PHARMACOLOGICAL AND DEVELOPMENTAL PROPERTIES OF ATP-SENSITIVE K+ CHANNELS IN CULTURED MYOTOMAL MUSCLE-CELLS FROM XENOPUS EMBRYOS

Unlike mammalian muscle cells in culture, cultured myotomal muscle cells of Xenopus embryos express ATP-sensitive K+ (K-ATP) channels; The K-ATP channels are blocked by internal ATP (half-maximal inhibition K-0.5 = 16 mu M) and to a lesser extent by internal ADP, are voltage independent, have an inward rectification at positive potentials and are inhibited by glibenclamide (K-0.5 = 2 mu M). Surprisingly, these K-ATP channels are not sensitive to K+ channel openers such as cromakalim. Opening of these K-ATP channels does not occur under normal physiological conditions. It is elicited by metabolic exhaustion of the muscle cell and it precedes the development of an irreversible rigor state. Neither intracellular acidosis nor an increase of intracellular Ca2+ are involved in K-ATP channel opening. Different types of K+ channels are successively expressed after plating of myotomal muscle cells. (I) sustained delayed-rectifier K+ channels; (2) K-ATP channels; (3) inward-rectifier K+ channels; (4) transient delayed-rectifier K+ channels. The current density associated with K-ATP channels far exceeds that of voltage-dependent K+ channels. Innervation controls the expression of these K-ATP channels. Go-culture of muscle cells with neurons from the neural tube decreases the number of active K-ATP channels per patch. Similarly, in situ innervated submaxillaris muscle of tadpoles at stage 50-55 has a very low density of K-ATP channels.