Myocardial contractility recovery during hypercapnic acidosis: its dissociation from recovery in pHi by ryanodine

Myocardial contractility falls quickly during respiratory acidosis but if acidosis is maintained a slow gradual return towards control state is detected. In cat papillary muscle, changes in developed tension (DT) during isometric contractions (pacing rate 0.2 Hz) and intracellular pH (pHi) were continuously monitored before and during hypercapnia to study the contribution of pHi recovery to the recovery of contractility. On exposure to hypercapnia (extracellular pH [pHo] = 6.90) DT fell to 50.33 +/- 2.20% of control and pHi decreased from 7.21 +/- 0.05 to 6.90 +/- 0.02. After 30 mins of hypercapnia DT recovered to 64.66 +/- 4.05% of control, but no significant recovery in pHi was detected. Intracellular sodium concentration slowly rose to 61.05 +/- 23.79% over basal level 10 mins after the onset of hypercapnia and it remained elevated for 10 mins before gradually returning to control levels. When pHo was kept at 7.40 during hypercapnia by increasing sodium bicarbonate concentration, DT recovered to 79.11 +/- 6.94% of control after 30 mins of hypercapnia, while a significant recovery of pHi (0.12 +/- 0.02 pH units) was detected. Low extracellular sodium concentration diminished contractility recovery during hypercapnia without changing the initial decrease in DT. 5-[N-ethyl-N-isopropyl] amiloride (EIPA) (5 microM) increased the initial fall in DT to 34.33 +/- 8.68% of control and abolished the recovery. Sarcoplasmic reticulum (SR) inhibition by ryanodine (0.5 microM) markedly reduced the recovery of contractility without altering the recovery in pHi.(ABSTRACT TRUNCATED AT 250 WORDS)