Effects of triiodothyronine on retention of beta-adrenergic responsiveness of voltage-gated transmembrane calcium current during culture of ventricular myocytes from neonatal rabbits.

To study the effects of triiodothyronine (T3) on responsiveness of L-type calcium currents to beta-adrenergic stimulation in neonatal hearts, ventricular myocytes were isolated from neonatal rabbits and cultured in medium containing 10% fetal bovine serum to which T3 had been added to achieve either hypothyroid, euthyroid, or hyperthyroid conditions, as assessed by measurement of free T3 concentrations. During a 24-h culture period, the striated rod-shaped myocardial cells progressively assumed a stellate shape with reduced surface area; however, the rate constants for diffusion of Na+ from a microelectrode pipette into the cells remained unchanged. Voltage-dependent characteristics of L-type calcium currents as assessed by whole-cell voltage clamp studies were also unchanged after culture with various concentrations of free T3. By contrast, the stimulation of voltage-gated transmembrane calcium current from baseline by isoproterenol was reduced (p < 0.05) in hypothyroid cells (15 +/- 8%; n = 14) compared with either euthyroid (86 +/- 15%; n = 18), hyperthyroid (54 +/- 16%; n = 12) or freshly isolated (50 +/- 12%; n = 10) myocytes. The differences in beta-adrenergic responsiveness of voltage-gated transmembrane calcium current to isoproterenol between euthyroid, hyperthyroid, and freshly isolated cells were not significant (p > 0.05). These results indicate that retention of beta-adrenergic responsiveness of voltage-gated transmembrane calcium current in neonatal cardiac myocytes depends on physiologic amounts of active thyroid hormone. Our culture method for neonatal cardiac myocytes will be useful for studying physiologic modulation of beta-adrenergic responsiveness.