The Calcium Pumping ATPase of Heart Plasma Membrane
At each beat, Ca2+ penetrates into heart cells through a gated channel. According to the prevailing theory, this promotes the release of a much larger amount of Ca2+ from sarcoplasmic reticulum to activate contraction. It is immediately evident that the trigger Ca2+ that has entered the cell must be reexported to prevent cellular Ca2+ overload. Since Ca2+ in the cytosol of heart cells is µM or less, whereas in the extracellular spaces it is mM, re-export evidently requires active transport. Two systems are responsible for it, a Na+/Ca2+ exchanger and a specific ATPase: they work in parallel, but have different kinetic properties, which qualify them for different roles. The ATPase has high Ca2+ affinity but low pumping rate, and can thus be considered as the fine tuner of cell Ca2+, which functions in heart in the same way as in all other cells where it has been demonstrated. The Na+/Ca2+ exchanger has lower Ca2+ affinity but high transport rate, and may thus be assumed to function at peak activation, when large amounts of Ca2+ must be ejected, but high affinity is not necessarily required due to the presumably increased concentration of Ca2+ at the inner side of the sarcolemma.
KeywordsATPase Activity Sarcoplasmic Reticulum Acidic Phospholipid Couple Enzyme Assay Cardiac Sarcolemma
Unable to display preview. Download preview PDF.