SUMMARY The surface of neonatal rat cells in culture, neonatal rat hearts, and adult rabbit hearts have qualitatively similar responses to lanthanum, ruthenium red, and colloidal iron stains. All demonstrate a surface coat and external lamina with abundant negatively charged sites. Cells with intact surface structure do not permit entry of lanthanum (La 3+ ) intracellularly. The surface of all the myocardial cells studied contained abundant sialic acid distributed in two distinct layers, one in the surface coat next to the lipid bilayer, the other in the external lamina at the interstitial interface. The removal of sialic acid from the cellular surface increases calcium (Ca 2+ ) exchangeability 5-to 6-fold. Its removal also permits La 3+ to enter the cell and displace more than 80% of cellular Ca 2+ . Despite these marked alterations in Ca 2+ and La 3+ permeability, sialic acid removal has no effect on potassium

Specific removal of sialic acid from cultured heart cells with purified neuraminidase increases cellular calcium exchangeability. Potassium exchange is unaffected or slightly decreased. Sialic acid removal also permits lanthanum, normally restricted to the cellular surface, to enter the cells and displace more than 80 percent of cellular calcium. The results indicate a specific role of cellular surface components in the control of calcium exchangeability in the heart.

No abstract

The effects of two excitation-contraction uncoupling agents, manganese (Mn) and verapamil, are compared in heart and fast-twitch skeletal muscle. Particular attention is given to the effect of the agents on the first contraction following a period of quiescence when the agents are administered during the quiescent period. Mn significantly diminishes dP/dt of the first beat in heart muscle, whereas verapamil has no significant effect. Neither Mn nor verapamil has a significant effect on the first postquiescent contraction in skeletal muscle, though verapamil produces a diminution of dP/dt in subsequent contractions in both tissues. A comparison of the effect of the agents on 45Ca exchange in heart cells in tissue culture indicates that Mn induces a rapid displacement of a rapidly exchangeable component of heart-cell calcium with subsequent inhibiton of influx. Verapamil, by contrast, produces no rapid displacement but only an inhibiton of influx. The functional and Ca-exchange effect of the two agents are compatible with a model which places most of the contractile-dependent Ca at the cellular surface in heart muscle and at deeper, intracellular sites in fast-twitch skeletal muscle.