Use of aequorin for the appraisal of the hypothesis of the release of calcium from the sarcoplasmic reticulum induced by a change of pH in skinned cardiac cells

Abstract

Listen

Translate article

A change of pH did not modify the sensitivity of aequorin to Ca 2+, but an increase of pH enhanced the Ca 2+ sensitivity of the myofilaments of a skinned canine cardiac Purkinje cell. The tension-pCa curve did not present any hysteresis when a given [free Ca 2+] was reached from a higher versus from a lower [free Ca 2+] in the presence of pH 6.60, 7.10 or 7.40. A rapid variation of pH in either direction failed to induce Ca 2+ release from the sarcoplasmic reticulum (SR). The proton ionophores CCCP and gramicidin also failed to induce Ca 2+ release from the SR. Increase of pH from 7.10 to 7.40 enhanced Ca 2+ accumulation into the SR and, there-by, augmented the Ca 2+ content of the SR. Consequently, the amplitude of a subsequent Ca 2+ release triggered by a rapid increase of [free Ca 2+] at the outer surface of the SR was increased. Conversely, a decrease of pH from 7.10 to 6.60 diminished the Ca 2+ accumulation into the SR, the Ca 2+ content of the SR and the amplitude of a subsequent Ca 2+-induced release of Ca 2+ from the SR. In addition, the optimum [free Ca 2+] for triggering Ca 2+-induced release of Ca 2+ was shifted to higher [free Ca 2+] values by a decrease of pH from 7.40 to 7.10 or 7.10 to 6.60. This may help to explain the enhancement of the aequorin light transient during acidosis in the intact cardiac muscle inasmuch as acidosis may increase the [free Ca 2+] trigger at the outer surface of the SR by inhibiting Na +-Ca 2+ exchange across the sarcolemma.