A QUANTITATIVE STUDY OF THE RELATION BETWEEN INTRACELLULAR PH AND FORCE IN RAT MESENTERIC VASCULAR SMOOTH-MUSCLE

Strips of rat mesenteric artery were loaded with carboxy-seminaphthorhodafluor (SNARF) to measure intracellular pH (pH(i)) and force simultaneously. pH(i) was altered by using weak acids and bases. Alkalinization produced an increase in force. For equal elevations of pH(i) a greater and faster increase of force was obtained in depolarized (high K+) than in non-depolarised preparations. Acidification produced little change in force unless the tissue was contracted (high-K+), in which case it elicited relaxation. Examination of the relationship between pH(i) and force in depolarized preparations showed that acidification produced a greater change in force than alkalinization. Removal of weak bases produced a transient acidification that was accompanied by a fall in force in all preparations. This was followed by a secondary contraction in depolarized preparations during the period over which pH(i) was acidic and being restored to resting values. Some preparations demonstrated a hysteresis in the relation between pH, and force. It is concluded that the relationship between pi-Ii and force in mesenteric vascular smooth muscle is not constant but depends on the previous history of the preparation, and may involve differences in the interactions between H+, Ca2+ and the contractile machinery.