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 a n alpha toxin permeabilised phasic (rat portal vein) and tonic (human umbilical artery) smooth muscle. Methods: Strips of the two muscles we re permeabilised using crude cr toxin from Staphylococcus aureus (2 mg .ml(-1)). The relationship between [Ca2+] and tension was then examine d at different bathing pH values. [Ca2+] was monitored using indo-1 fl uorescence. Results: GTP-gamma-S (100 mu M) potentiated maximum calciu m activated force in rat portal vein on average to 146%, but had no si gnificant 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 incre ased maximum calcium activated force in rat portal vein. Raising bathi ng pH from 7.2 to 7.7 depressed both submaximal and maximum calcium ac tivated force in rat portal vein. Altered bathing pH had no significan t effect on either maximum or submaximal calcium activated force in hu man umbilical artery. The calcium sensitivity of tension production wa s not significantly effected by acidic pH in either preparation. Howev er, alkaline pH caused a similar fall in the calcium sensitivity in bo th preparations. Conclusions: Tension generated from the contractile p roteins of a phasic smooth muscle (rat portal vein) are more sensitive to altered bathing pH than those of a tonic smooth muscle (human umbi lical artery).