Calcium and Monovalent Ions in Smooth Muscle

Electron probe analysis has shown that the high Cl and K contents of smooth muscle reflect the generalized cytoplasmic distribution of these elements and are not due to sequestration in organelles. These findings, in agreement with other studies, indicate that ECl is less and EK more negative than the membrane potential. The susceptibility of cellular Na content to preparatory procedures has been confirmed. Nuclei do not sequester Na, and transmitochondrial monovalent ion (Na, K, Cl) gradients are very small or nonexistent. The major identified intracellular store of Ca is the sarcoplasmic reticulum; mitochondrial Ca is low in resting as well as in maximally contracted normal portal vein smooth muscle. Observations on mitochondrial loading in saponin-skinned smooth muscle suggest that in intact cells maximal contraction may take place at less than 10(-5) M free cytoplasmic Ca2+ concentrations [Ca2+]. Problems of the composition of abnormal (hypertrophied, hypertensive, and atherosclerotic) smooth muscles are considered.