Acetylcholine elicits a rebound stimulation of Ca2+ current mediated by pertussis toxin-sensitive G protein and cAMP-dependent protein kinase A in atrial myocytes.

Abstract

Cholinergic inhibition of atrial contraction is typically followed by a rebound positive inotropic response. In the present study, we used a nystatin-perforated patch whole-cell recording method to determine whether acetylcholine (ACh) elicits a rebound stimulation of L-type Ca2+ current (ICa,L) in cat atrial myocytes. ACh (1 mumol/L) decreased basal ICa,L (-19 +/- 2%). Within approximately 30 s of returning to ACh-free solution, basal ICa,L exhibited a rebound increase above the control level (+61 +/- 7%) that returned to the control level within 4 to 5 minutes. ACh elicited concomitant changes in cell shortening, ie, a decrease followed by a rebound increase. The EC50 and maximal response of ACh-induced inhibition and rebound stimulation of ICa,L were 1.9 x 10(-9) mol/L and -30%, respectively, and 2.9 x 10(-8) mol/L and +64%, respectively. All effects of ACh on ICa,L were blocked by prior exposure to 1 mumol/L atropine or 100 mumol/L AFDX116 and unaffected by 0.2 mumol/L pirenzepine or 1 mumol/L propranolol. In the presence of ACh, exposure to atropine elicited stimulation of ICa,L.ACh-induced inhibition and rebound stimulation of current were independent of external Ca2+. Rebound stimulation of ICa,L was associated with a negative shift in the voltage dependence of ICa,L activation. Inhibition of protein kinase A by 50 mumol/L Rp-cAMPs decreased basal ICa,L by 36 +/- 1% and abolished the rebound stimulation of ICa,L. Forskolin (0.01 mumol/L) or isoproterenol (0.01 mumol/L) had no effect on basal ICa,L, but each accentuated the rebound increase in ICa,L. When adenylate cyclase was maximally stimulated with 1 mumol/L isoproterenol plus 2 mumol/L forskolin, ACh decreased ICa,L but failed to elicit rebound stimulation of ICa,L. Milrinone (10 mumol/L) increased basal ICa,L by 70 +/- 7% and significantly attenuated the rebound stimulation of ICa,L. Exposure to 1 mmol/L 8-bromo-cGMP elicited a small decrease in basal ICa,L, attenuated ACh-induced inhibition, and enhanced the rebound stimulation of ICa,L. Incubation in pertussis toxin prevented all ACh-induced changes in ICa,L. Inhibition of nitric oxide synthase by 100 mumol/L NG-monomethyl-L-arginine (L-NMMA) decreased basal ICa,L by -20 +/- 5%, prevented ACh-induced inhibition, and markedly attenuated the rebound stimulation of ICa,L. We conclude that in cat atrial myocytes ACh acts via M2 muscarinic receptors and pertussis toxin-sensitive G protein to inhibit basal ICa,L and that on withdrawal ACh elicits a rebound stimulation of ICa,L. Rebound stimulation of ICa,L is mediated via cAMP-dependent protein kinase A enhanced by ACh-induced inhibition of phosphodiesterase.(ABSTRACT TRUNCATED AT 400 WORDS)