A COMPARISON OF THE EFFECTS OF INTRACELLULAR AND EXTRACELLULAR PH ON CONTRACTION IN ISOLATED RAT PORTAL-VEIN

1. The effects of changes in extracellular and intracellular pH on spo ntaneous contractile activity in isolated rat portal vein have been in vestigated. 2. Small strips of portal vein were loaded with the pH-sen sitive fluorophore carboxy-SNARF and intracellular pH (pH(i)) and cont raction were measured simultaneously at 37-degrees-C. The tissue was s uperfused with oxygenated, Hepes-buffered solutions at pH 7.4. Intrace llular pH was altered by isosmotic substitution of weak acids or bases . External pH (pH(o)) was altered by addition of strong acid or base t o the solution. 3. The mean resting value of pH(i) was 7.06 +/- 0.03 ( n = 28). Alteration of pH(i) led to changes in spontaneous activity. A ddition of butyrate (20 mm) reduced pH(i) by 0.18 +/- 0.01 pH units (n = 8). Decreasing pH(i) produced an early, brief increase in contracti le activity followed by a longer lasting decrease or even abolition of contraction. 4. Addition of 20 mm trimethylamine or NH4Cl increased p H(i) by around 0.2 pH units and produced an early transient decrease i n contractile activity followed by a later maintained increase, both i n frequency and magnitude. Removal of base produced a rapid rebound de crease in pH(i) which was associated with a further transient increase in contractile activity followed by decreased activity. The effects o f base on both pH(i) and contraction were concentration dependent over the range investigated (2.5-30 mM) 5. Alteration of pH(o) produced a change in pH(i) in the portal vein. The pH(i) change was rapid compare d to other non-vascular cells (about 1 min to half-maximal response). A 0.5 pH unit change in pH(o) produced a change in pH(i) of around 0.1 6 pH units, a value similar to that found in most other cells. The eff ects of changing pH(o) on contractile activity of the portal vein were less than those obtained by changing pH(i) alone, even when the pH(o) induced pH(i) changes were of similar magnitude. When pH(o) did alter contractile activity, decreasing pH(o) decreased spontaneous contracti le activity and increasing pH(o) increased it. Changing pH(o) never pr oduced the early transient effects on contraction seen with alteration of pH(i). 6. The contractile responses of the portal vein to changes of pH(i) may be functionally important. For example, the decrease in c ontractile activity with decreased pH(i) may contribute to the fall of force observed in hypoxia. The relationship between pH and contractio n in portal vein is discussed and compared to that found in other smoo th muscles and in permeabilized preparations.