Brief Reviews Exchange of Calcium Ions in the Mammalian Myocardium MECHANISMS AND PHYSIOLOGICAL SIGNIFICANCE

• Calcium ions are essential for coupling be-tween excitation of the cardiac cell membrane and activation of the contractile proteins (excita-tion-contraction coupling). This process can be summarized in the following way (1-3). During membrane depolarization, i.e., during the cardiac action potential, calcium (Ca) ions flow across the membrane into the cardiac muscle fiber. This influx directly and indirectly (possibly by a regenerative release of Ca from the sarcoplasmic reticulum) causes an increase in the concentra-tion of free Ca ions in the myoplasm. In the relaxed muscle, this concentration is on the order of 10~7M. During excitation, the free myoplasmic Ca concentration increases by one to two orders of magnitude, which permits binding of Ca ions to the protein troponin and ultimately leads to a contraction of the myofibrils. However, a trans-port system of the sarcoplasmic reticulum with a high affinity for Ca ions reduces the free myo-plasmic Ca concentration again and therefore causes relaxation of the contractile proteins. Ca ions are then stored in the sarcoplasmic reticulum. To avoid an overload of the sarco-plasmic reticulum and the mitochondria with Ca, these ions are pumped out of the cell across the cell membrane (sarcolemma) apparently by a transport mechanism involving exchange of external sodium (Na) ions for internal Ca ions (Na-Ca exchange). Obviously, the sarcolemma plays an important role in the regulation of intra-cellular Ca concentration. Only recently, some information has become available about the mechanisms of inward Ca movement during the action potential and outward Ca transport (4-6). The following discussion summarizes the available information about the mechanisms of these Ca movements across the sarcolemma of cardiac cells. Also some basic relations between Ca movements and contractile activation are presented