EFFECT OF ALTERED BATHING PH ON CALCIUM-ACTIVATED FORCE IN ALPHA-TOXIN-PERMEABILIZED RAT PORTAL-VEIN AND HUMAN UMBILICAL ARTERY

Objective: The aim was to investigate the effect of altered bathing pH on calcium activated force generated by the contractile proteins in an alpha toxin permeabilised phasic (rat portal vein) and tonic (human umbilical artery) smooth muscle. Methods: Strips of the two muscles were permeabilised using crude cr toxin from Staphylococcus aureus (2 mg.ml(-1)). The relationship between [Ca2+] and tension was then examined at different bathing pH values. [Ca2+] was monitored using indo-1 fluorescence. Results: GTP-gamma-S (100 mu M) potentiated maximum calcium activated force in rat portal vein on average to 146%, but had no significant effect on human umbilical artery, confirming the difference in contractile behaviour between the two muscles. Lowering bathing pH from 7.2 to 6.7 depressed submaximal calcium activated force and increased maximum calcium activated force in rat portal vein. Raising bathing pH from 7.2 to 7.7 depressed both submaximal and maximum calcium activated force in rat portal vein. Altered bathing pH had no significant effect on either maximum or submaximal calcium activated force in human umbilical artery. The calcium sensitivity of tension production was not significantly effected by acidic pH in either preparation. However, alkaline pH caused a similar fall in the calcium sensitivity in both preparations. Conclusions: Tension generated from the contractile proteins of a phasic smooth muscle (rat portal vein) are more sensitive to altered bathing pH than those of a tonic smooth muscle (human umbilical artery).