Force, membrane potential, and [Ca2+]i during activation of rat mesenteric small arteries with norepinephrine, potassium, aluminum fluoride, and phorbol ester. Effects of changes in pHi.

In activated rat mesenteric small arteries, the effect of pHi on force, membrane potential, and free cytosolic calcium ([Ca2+]i) was assessed. Arteries were mounted in a myograph for isometric force development, and [Ca2+]i, pHi, or membrane potential was measured simultaneously with force. During activation with norepinephrine, potassium, aluminum fluoride (AlF-4), and phorbol 12-myristate 13-acetate (PMA, a phorbol ester), the vessels depolarized and [Ca2+]i increased, although the ratio of force to [Ca2+]i was less during potassium activation than with the other types of activation. Changes in pHi, with a constant pHo, were induced with NH4Cl or by changing PCO2. In resting vessels, the effects of the changes in pHi on tension, membrane potential, and [Ca2+]i were negligible. In vessels activated with norepinephrine or AlF-4, alkalinization caused an acute decrease of tone, which could be explained by a decrease in [Ca2+]i consequent to repolarization of the membrane. In vessels activated with potassium or PMA, the effects of alkalinization were smaller. This is consistent with acute alkalinization, affecting steps proximal in the excitation-contraction coupling distal to activation of G proteins. Acidification caused a transient increase in tone and [Ca2+]i, irrespective of the mode of stimulation, without affecting the membrane potential. Ryanodine did not abolish the transient increase in tone and [Ca2+]i. Thus, acute intracellular acidification may induce tone by release of an intracellular ryanodine-insensitive calcium pool or by affecting transmembranal calcium flux although in a membrane potential-independent way.