Energy cost of membrane depolarization in hog carotid artery

Past studies have shown that during stable stepwise activations of vascular smooth muscle with varying concentrations of high-K+, both cytoplasmic free-Ca++ and membrane depolarization vary. On the other hand, during stepwise activations with varying concentrations of external Ca++ in the presence of constant external Ca++ in the presence of constant external depolarizing high-K+, cytoplasmic free-Ca++ varies, while membrane depolarization remains relatively constant. In this study, the rates at which suprabasal energy metabolism (oxygen consumption and lactic acid production) increased with increasing isometric tension maintenance were measured under both circumstances. Suprabasal energy metabolism with increasing membrane depolarization (increasing external K+) exceeded that with constant depolarizing-[K+] and varying Ca++ by less than 2.5% at all levels of activation, which was not statistically significant (P greater than 0.70). We conclude therefore that the steady state metabolic energy cost of membrane depolarization per se during contractile activity in vascular smooth muscle from a tonic conducting vessel (hog carotid artery) is negligible. Although the possibility cannot be excluded, we find no metabolic evidence that increased cytoplasmic free-Ca++ itself activates an ATPase associated with Ca++ sequestration and/or extrusion beyond that present in the relaxed state. Activation of hog carotid artery with an isosmotic K+-for-Na+-substituted medium fails to stimulate aerobic glycolysis at all levels of K+ substitution. Experiments were performed at the muscle length optimal for isometric tension generation and at 37 degrees C.