Effects of norepinephrine and isopentenyladenosine on Na+/Ca2+ exchange currents in isolated guinea pig ventricular myocytes.

AIM: To study the effects of norepinephrine (NE) and isopentenyladenosine (Iso) on Na+/Ca2+ exchange currents and the receptor mechanism. METHODS: The quasi-steady state current-voltage relationship from the isolated guinea pig ventricular myocytes was measured using whole-cell voltage-clamp techniques with a ramp pulse protocol. RESULTS: At potential of +50 mV, NE 0.005, 0.05, and 5 mumol.L-1 increased the Ni(2+)-sensitive current by 29% +/- 9%, 72% +/- 11%, and 124% +/- 31.4%, respectively; Iso 1.5, 150, and 1500 nmol.L-1 caused increases in the Ni(2+)-sensitive current by 2.8% +/- 2.8%, 56% +/- 13%, and 102% +/- 12%, respectively. Propranolol 10 mumol.L-1 completely inhibited the current changes induced by NE and Iso while phentolamine 50 mumol.L-1 showed no effects. CONCLUSION: NE and Iso increased the Na+/Ca2+ exchange currents via stimulation of cardiac beta-adrenoceptor.