[Acid-base balance and contraction of the cardiac muscle]

In cardiac muscle, intracelular pH (pHi) is regulated to close to 7.1, both by membrane transport systems (Na-H and Cl-HCO3 exchange) and intracellular H+ buffers. Internal acidosis depresses the contraction, while alkalosis increases it. Internal proton depresses the contraction even without altering the internal concentration of Ca (Cai). The proton blocks the Ca channels. However, as it releases membrane-bound Ca and the influx of Na via Na-H exchange increases Ca influx through Na-Ca exchange, pHi decrease is often accompanied by the Cai increases. The internal proton also affects the uptake and release of sarcoplasmic reticulum, Ca binding to troponin, actomyosin dynamics and variable catalytic processes.