Temperature- and Mg-ATP-dependent regulation of Ca2+ sensitivity of smooth muscle actomyosin ATPase.

Many smooth muscles on metabolic depletion undergo a contraction that is insensitive to EGTA [ethylene glycol-bis (beta-aminoethylether)N,N-tetraacetic acid]. Chicken gizzard actomyosin shows a progressive loss of Ca sensitivity accompanied by activation of EGTA-Mg-ATPase at temperatures near 37 degrees C with decreasing ATP concentrations. Ca2+-dependent phosphorylation still occurs under these conditions when the ATPase is Ca insensitive. Activation of EGTA-Mg-ATPase at low ATP concentration is not due to a pseudo-ATPase, or due to denautration of the actomyosin at 37 degrees C. Magnesium concentrations above 1 mM are required for observing the enhanced EGTA-Mg-ATPase activity and the Ca sensitivity is very markedly influenced by the magnesium concentrations of medium at low ATP. When the Mg-to-ATP ratio (5:1) was kept constant for varying ATP concentrations, activation of EGTA-ATPase was not observed. This activation was not due to the characteristics of the ATP regenerating system (phosphoenolpyruvate and pyruvate kinase) because with phosphocreatine and creatine phosphokinase similar results were obtained. Thus the EGTA-insensitive rise in tension during metabolic depletion is due to activation of Mg-ATPase and loss of Ca sensitivity at 37 degrees C, a temperature at which mammalian smooth muscles normally function.