ENDOTHELIN-1 ENHANCES CALCIUM ENTRY THROUGH T-TYPE CALCIUM CHANNELS IN CULTURED NEONATAL RAT VENTRICULAR MYOCYTES

Endothelin-1 (ET-1), a 21-amino acid vasoconstrictive peptide, increases intracellular Ca2+ level and has hypertrophic action on ventricular myocytes. To elucidate a possible role of Ca2+ entry through sarcolemmal Ca2+ channels on this ET-1 action, we examined effects of ET-1 on L-type (I(Ca,L)) and T-type (I(Ca,T)) Ca2+ currents in cultured neonatal rat ventricular myocytes using the patch-clamp technique. ET-1 at a concentration of 10 nM increased the maximum current density of I(Ca,T) from -3.0+/-1.4 muA/cm2 in the control condition to -4.4+/-1.6 muA/CM2 (p<0.01). Although the peak amplitude of I(Ca,L) was decreased during ET-1 application (from -9.7+/-1.9 muA/cm2 in the control condition to -5.0+/-1.4 muA/cm2 [p<0.01]), this magnitude of decrease in I(Ca,L) (52 t 19%) was comparable to that of spontaneous "run-down" of I(Ca,L) (47+/-26%). The enhancement of I(Ca,T) by ET-1 was dose dependent; it was initiated as low as 032 nM, and the maximal response was attained at approximately 10 nM, with a half-maximal dose of 1.26 nM. The enhancement of I(Ca,T) by ET-1 was antagonized by protein kinase C inhibitors staurosporine (0.2 muM) and 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7, 20 muM) applied to the pipette solution. Extracellular application of tumor-promoting phorbol esters, phorbol 12,13-dibutyrate (PDBu) and 4beta-phorbol 12-myristate 13-acetate, augmented I(Ca,T), PDBu (0.2 muM) increased the maximal current density of I(Ca,T) from -4.2+/-0.5 muA/cm2 in the control condition to -5.5+/-1.0 muA/cm2 (p<0.01). In the presence of H-7 (20 muM) in the pipette solution, PDBu failed to enhance I(Ca,T), and an inactive isomer of PDBu (4alpha-phorbol 12,13-dibutyrate, 0.2 muM) did not augment I(Ca,T). Thus, ET-1 enhances Ca2+ entry through the sarcolemmal T-type Ca2+ channel, possibly through a pathway involving activation of protein kinase C. This ET-1 action may be involved in the rise of the intracellular Ca2+ level and may contribute to the induction of cardiac hypertrophy by ET-1.