NADH fluorescence and spontaneous activity of rat portal veins at decreased pH

(1) Intracellular fluorescence of pyridine nucleotides (PN) of smooth muscle cells from rat portal veins was measured simultaneously with mechanical force development at different acidotic pH in order to relate cell metabolism to spontaneous contractile activity. (2) The pH was lowered (I) by increasing the pCO2 and (II) by decreasing the HCO3--concentration at constant pCO2 in the perfusing solution, producing respiratory and metabolic acidosis respectively. (3) Under both acidotic conditions, decreased force development and frequency of the spontaneous activity was paralleled by alterations of energy metabolism indicated by a shift of the pyridine nucleotide redox-state towards oxidation. (4) Equilibrium values of intracellular PN fluorescence and contraction force development differed under respiratory and metabolic acidosis; spontaneous contraction frequency, regarded as an indicator of pacemaker activity, did not show a significant difference under either acidotic condition. (5) During respiratory acidosis the spontaneous activity exhibited a typically time-dependent behaviour. Since CO2 is thought to freely penetrate cell membranes, this transient break of the mechanical activity may be explained in view of a phasic change of intracellular pH.