THE RESTRICTION OF DIFFUSION OF CATIONS AT THE EXTERNAL SURFACE OF CARDIAC MYOCYTES VARIES BETWEEN SPECIES

In cardiac muscle sarcolemmal structures such as T-tubules, caveolae a nd negatively charged protein-polysaccharides may affect the rate of c ation exchange on the external surface of the cells. To test this hypo thesis, we examined the rate of external cation exchange in adult rabb it and rat ventricular myocytes using a rapid solution switcher to cha nge the bulk external solution within 4 ms. To assess the rate of diff usion of monovalent cations, we increased [K+](o) from 4.4 to 6.6 or 8 .8 mM and measured the time required to achieve a stable membrane depo larization. In rat myocytes, the mean time to 90% depolarization (t(90 )) was significantly longer than that in rabbit myocytes (137 and 64 m s, respectively) and the difference in t(90) was not associated with t he cell size, To assess the time course of exchange of external Ca2+, we rapidly exposed the myocytes to O Ca2+-2 mM EGTA solution at specif ic time points before action potentials or voltage clamp steps, and me asured the rate of alteration of the normalized peak [Ca2+] transient (Fluo-3) or Ca2+ current. Exposure to O Ca2+-2 mM EGTA solution caused a decline in the intracellular calcium transient. In rat myocytes, th e rate of decline in the [Ca2+], transient was much slower (t(90) > 15 00 ms, the time required for 90% decline) than for the rabbit (t(90)= 295 ms). Also, the rate of decline in the Ca2+ current was prolonged i n rat myocytes (t(90) = 910 ms) compared with rabbit myocytes (t(90) = 241 ms). These data indicate that there is a restricted space on the external surface of sarcolemma which limits diffusion of divalent cati ons more markedly than monovalent cations, The extent of this limitati on of cation diffusion varies between species, and may have functional significance.