RESTORATION OF JUNCTIONAL TETRADS IN DYSGENIC MYOTUBES BY DIHYDROPYRIDINE RECEPTOR CDNA

Excitation-contraction coupling was restored in primary cultures of dy sgenic myotubes by transfecting the cells with an expression plasmid e ncoding the rabbit skeletal muscle dihydropyridine receptor. Dishes co ntaining normal, dysgenic, and transfected myotubes were fixed, freeze -fractured, and replicated for electron microscopy. Numerous small dom ains in the surface membrane of normal myotubes contain ordered arrays of intramembrane particles in groups of four (tetrads). The dispositi on of tetrads in the arrays is consistent with alternate positioning o f tetrads relative to the underlying feet of the sarcoplasmic reticulu m. Dysgenic myotubes have no arrays of tetrads. Some myotubes from suc cessfully transfected cultures have arrays of tetrads with spacings eq ual to those found in normal myotubes. Thus the dihydropyridine recept or appears to be needed for the formation of tetrads and their associa tion with the sarcoplasmic reticulum feet. This result is consistent w ith the hypothesis that each tetrad is composed of four dihydropyridin e receptors.