Differences in isoproterenol stimulation of Ca2+ current of rat ventricular myocytes in neonatal compared to adult.
The developmental changes in the isoproterenol stimulation of the L-type calcium current (ICa(L)) were studied in freshly isolated neonatal (3-5-day-old) and adult (2-3-month-old) rat ventricular myocytes using whole-cell voltage clamp (at room temperature). ICa(L) was measured as the peak inward current at a test potential of +10 mV (or +20 mV) by applying a 300 ms pulse from a holding potential of -40 mV. The pipette solution was Cs(+)-rich and Ca(2+)-free. The external solution was Na(+)-free and K(+)-free. Isoproterenol stimulated ICa(L) in a dose-dependent manner. The concentrations of isoproterenol for half-maximal effect were 6.8 nM in neonatal and 13.3 nM in adult. The maximal stimulation of ICa(L) was 147 +/- 14% in neonatal and 97 +/- 7% in adult. The steady-state inactivation curves were not affected by isoproterenol, whereas the steady-state activation curve was shifted to the left in both neonatal and adult. Forskolin (10 microM) increased ICa(L) by 105 +/- 10% in neonatal and 90 +/- 12% in adult. After stimulating ICa(L) by forskolin, the addition of isoproterenol produced a further increase of ICa(L) by 99 +/- 27% in neonatal, but only by 19 +/- 3% in adult. The presence of an inhibitor of cAMP-dependent protein kinase in the pipette did not affect this marked difference between neonatal (87 +/- 23%) and adult (11 +/- 8%). We conclude that, in rat ventricular myocytes, (1) stimulation of ICa(L) by the beta-adrenoceptor agonist, isoproterenol, is already fully developed in the neonatal stage and actually decreases during development; (2) there is evidence for a cAMP-independent stimulation of Ca2+ channels by isoproterenol, and this is greater in neonatal than in adult. We believe that the cAMP-independent pathway is the direct pathway mediated by Gs alpha protein.
Katsube
Y
Y
Department of Molecular and Cellular Physiology, College of Medicine, University of Cincinnati, OH 45267-0576, USA.
Yokoshiki
H
H
Nguyen
L
L
Sperelakis
N
N
D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000375:Aging / Q000378:metabolism* / Q000502:physiology; D000818:Animals; D000831:Animals, Newborn; D002120:Calcium Channel Agonists / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D005576:Colforsin / Q000494:pharmacology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006321:Heart / Q000187:drug effects / Q000254:growth & development; D066298:In Vitro Techniques; D007545:Isoproterenol / Q000494:pharmacology*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D051381:Rats; D017207:Rats, Sprague-Dawley
D000318:Adrenergic beta-Agonists; D002120:Calcium Channel Agonists; D015220:Calcium Channels; D004791:Enzyme Inhibitors; D005576:Colforsin; D017868:Cyclic AMP-Dependent Protein Kinases; D007545:Isoproterenol