THE DESENSITIZATION OF THE EMBRYONIC MOUSE MUSCLE ACETYLCHOLINE-RECEPTOR DEPENDS ON THE CELLULAR ENVIRONMENT

The rate of desensitization of nicotinic acetylcholine (ACh) receptor (nAChR), an important characteristic of nAChR function, was studied in myotubes of the mouse C2C12 cell line at different times after fusion, by measuring the decay of ACh-evoked currents (I-ACh) under various patch-clamp configurations. We observed a progressive slowing of I-ACh decay rate (half-decay time rose from about 0.5 s to over 5 s) in myotubes of increasing size (i.e. age) under all experimental conditions, except in outside-out patches, when I-ACh decayed as fast as in the smallest myotubes. Single-channel conductance (about 35 pS) and open time (about 3.5 ms), measured in outside-out and cell-attached patches, were independent of myotube size. In Xenopus oocytes injected with poly(A(+))RNA extracted from C2C12 myoblasts or mature myotubes, I-ACh decay was about 50 times slower than in myotubes. Neither cAMP-dependent nor diacylglycerol-dependent protein kinases, actin nor microtubule polymerization state influenced I-ACh decay. Our data indicate that the cellular environment, but not readily dialysable cytosolic factors, markedly influences the functional behaviour of nAChR.