BACKGROUNDFew studies have explored the inward sodium current (INa) kinetics of transitional cardiomyocytes. This study aimed to explore the kinetics of transitional cardiomyocytes types alpha and beta.

METHODSThe whole-cell patch clamp technique was used to study the rapid INa of isolated transitional cardiomyocytes in the Koch triangle of rabbit hearts.

RESULTSMaximal amplitude and density of INa in type alpha and type beta was (-1627 +/- 288) pA (alpha), (-35.17 +/- 6.56) pA/pF (beta) and (-3845 +/- 467) pA (alpha), (-65.64 +/- 10.23) pA/pF (beta) (P < 0.05). Steady state activation curves of INa, fitted to a Boltzmann distribution for both types, were sigmoid in shape. Half activation voltage and slope factors did not significantly differ between types at (-43.46 +/- 0.85) mV (alpha), (-41.39 +/- 0.47) mV (beta) or (9.04 +/- 0.66) mV (alpha), (11.08 +/- 0.89) mV (beta). Steady state inactivation curves of INa, fitted to a Boltzmann distribution in both types were inverse "S" shape. Half inactivation voltage and slope factors were (-109.9 +/- 0.62) mV (alpha), (-107.5 +/- 0.49) mV (beta) and (11.78 +/- 0.36) mV (alpha), (11.57 +/- 0.27) mV(beta), (P > 0.05), but time constants of inactivation were significantly different at (1.10 +/- 0.19) mV (alpha) and (2.37 +/- 0.33) ms (beta), (P < 0.05). Time constants of recovery from inactivation of INa for both types were (122.16 +/- 27.43) mV (alpha) and (103.84 +/- 28.97) ms (beta) (P < 0.05).

CONCLUSIONSTransitional cardiomyocytes in rabbit hearts show a heterogeneous, voltage gated and time dependent fast inward sodium current. Types alpha and beta show the features of INa similar to those in slow- and fast-response myocytes, with probably better automaticity and conductivity, respectively.