THE NA+/H+ ANTIPORT IS ACTIVATED BY SERUM AND PHORBOL ESTERS IN PROLIFERATING MYOBLASTS BUT NOT IN DIFFERENTIATED MYOTUBES - PROPERTIES OF THE ACTIVATION PROCESS

Abstract

The properties of the Na+/H+ exchange system were studied with 22Na+ uptake techniques at 2 stages of [rat] muscle development: proliferating myoblasts and differentiated myotubes. The characteristics of the interactions of the exchanger with external H+, with external Na+, and with amiloride or its more potent analogs are the same at both stages of development. Differences between the 2 stages of development concern: the internal pH (pHi) dependence of the Na+/H+ exchanger, and the activation of the Na+/H+ exchanger by serum and phorbol ester which is observed in myoblasts but not in myotubes. Properties of the Na+/H+ exchanger in myoblasts after serum activation seem to be identical to those observed in myotubes with or without serum as if myotube formation stabilized a fully activated state of the exchanger. The activation of the myoblast Na+/H+ exchange system by serum is due to a shift of the pHi dependence towards alkaline pHi values and to an increase in the maximal activity of the Na+/H+ exchange system at acidic pH. Phorbol esters which are well-known activators of protein kinase C can only partially mimic the effects of serum on the Na+/H+ exchanger: they produce a shift of the pH dependence, but they do not increase the maximal activity at acidic pH.