DEVELOPMENTAL-CHANGES IN THE EFFECT OF ACIDOSIS ON CONTRACTION, INTRACELLULAR PH, AND CALCIUM IN THE RABBIT MESENTERIC SMALL ARTERY

The purpose of the present study was to determine developmental change s in the effect of respiratory acidosis on vascular smooth muscle cont raction. Vessel diameter, intracellular pH (pH(i)), and calcium concen tration ([Ca](i)) were measured in a cannulated preparation of the sma ll mesenteric artery of newborn and adult rabbits. In the artery preco ntracted by high KCl, acidosis caused a vasorelaxation both in the new born and the adult; the vasorelaxation was greater in the newborn than in the adult. The fura-2 fluorescence ratio, an indicator of [Ca](i), decreased transiently during acidosis and the decrease was similar in the two age groups. In the artery precontracted by norepinephrine, ac idosis caused a transient vasoconstriction in the adult and a vasorela xation in the newborn. In these vessels, the fura-2 fluorescence ratio increased transiently during acidosis; the increase was similar in th e two groups. Upon induction of acidosis, pH(i) fell rapidly in the ar tery precontracted by norepinephrine or high KCl, and the depression o f pH(i) was similar in the two groups. In the skinned smooth muscle pr eparation, a tension-[Ca] relationship curve at pH 7.1 was not signifi cantly different from that at pH 6.8 in the adult. In the newborn, the tension-[Ca] curve at pH 6.8 was shifted to the right, compared with that at pH 7.1. These data suggest that the vasorelaxant effect of res piratory acidosis in the premature vessel is greater than in the adult . The greater vasorelaxation in the newborn cannot be explained by the age-related difference in pH(i) or [Ca](i) during acidosis. The great er sensitivity of myofibrils to low pH(i) in the newborn may, at least in part, be responsible for the greater vasorelaxation in this age gr oup.