ACETYLCHOLINE ELICITS A REBOUND STIMULATION OF CA2+ CURRENT MEDIATED BY PERTUSSIS-TOXIN-SENSITIVE G-PROTEIN AND CAMP-DEPENDENT PROTEIN-KINASE-A IN ATRIAL MYOCYTES

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 (I-Ca,I-L) in cat atrial myocytes. ACh (1 mu mol/L) decreased basal I-Ca,I-L (-19+/-2%). Within approximate to 30 s of returning to ACh-free solution, basal I-Ca,I-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 EC(50) and maximal response of ACh-induced inhibition and rebound stimulation of I-Ca,I-L were 1.9x10(-9) mol/L and -30%, respectively, and 2.9x10(-8) mol/L and +64%, respectively. All effects of ACh on I-Ca,I-L were blocked by prior exposure to 1 mu mol/L atropine or 100 mu mol/L AFDX116 and unaffected by 0.2 mu mol/L pirenzepine or 1 mu mol/L propranolol. In the presence of ACh, exposure to atropine elicited stimulation of I-Ca,I-L. ACh-induced inhibition and rebound stimulation of current were independent of external Ca2+. Rebound stimulation of I-Ca,I-L was associated with a negative shift in the voltage dependence of I-Ca,I-L activation. Inhibition of protein kinase A by 50 mu mol/L Rp-cAMPs decreased basal I-Ca,I-L by 36+/-1% and abolished the rebound stimulation of I-Ca,I-L. Forskolin (0.01 mu mol/L) or isoproterenol(0.01 mu mol/L) had no effect on basal I-Ca,I-L, but each accentuated the rebound increase in I-Ca,I-L. When adenylate cyclase was maximally stimulated with 1 mu mol/L isoproterenol plus 2 mu mol/L forskolin, ACh decreased I-Ca,I-L but failed to elicit rebound stimulation of I-Ca,I-L. Milrinone (10 mu mol/L) increased basal I-Ca,I-L, by 70+/-7% and significantly attenuated the rebound stimulation of I-Ca,I-L. Exposure to 1 mmol/L 8-bromo-cGMP elicited a small decrease in basal I-Ca,I-L, attenuated ACh-induced inhibition, and enhanced the rebound stimulation of I-Ca,I-L. Incubation in pertussis toxin prevented all ACh-induced changes in I-Ca,I-L. Inhibition of nitric oxide synthase by 100 mu mol/L N-G-monomethyl-L-arginine (L-NMMA) decreased basal I-Ca,I-L by -20+/-5%, prevented ACh-induced inhibition, and markedly attenuated the rebound stimulation of I-Ca,I-L. We conclude that in cat atrial myocytes ACh acts via M(2) muscarinic receptors and pertussis toxin-sensitive G protein to inhibit basal I-Ca,I-L and that on withdrawal ACh elicits a rebound stimulation of I-Ca,I-L. Rebound stimulation of I-Ca,I-L is mediated via cAMP-dependent protein kinase A enhanced by ACh-induced inhibition of phosphodiesterase. This mechanism contributes directly to the positive inotropic response that follows cholinergic inhibition of atrial contraction.