ALPHA-ADRENERGIC CONTROL OF VOLUME-REGULATED CL- CURRENTS IN RABBIT ATRIAL MYOCYTES - CHARACTERIZATION OF A NOVEL IONIC REGULATORY MECHANISM

alpha-Adrenergic stimulation is known to play a role in cardiac arrhyt hmogenesis and to modulate a variety of cardiac K+ currents. The effec ts of alpha-adrenergic stimulation on Cl- currents are largely unknown . Many cardiac cell types show a volume-sensitive Cl- current induced by cell swelling (I-Cl.swell). The present experiments were designed t o assess the potential alpha-adrenergic modulation of I-Cl.swell in ra bbit atrial myocytes. I-Cl.swell was induced with the use of a hypoton ic superfusate, under conditions designed to prevent currents carried by K+, Na+, and Ca2+ ions. A basal Cl- current (I-Cl.b) was observed u nder isotonic conditions in 128 of 150 cells (85%), had the same depen dency on [Cl-](o) as I-Cl.swell, and was reduced by cell shrinkage ind uced by hypertonic superfusion, suggesting that I-Cl.b is carried by t he same volume-sensitive Cl- conductance as I-Cl.swell. Phenylephrine produced a concentration-dependent and near-complete inhibition of I-C l.b and I-Cl.swell with EC(50) values of 86+/-5 and 72+/-7 (mean+/-SEM ) mu mol/L, respectively, at +20 mV. Norepinephrine (administered in t he presence of 1 mu mol/L propranolol) also inhibited I-Cl.b and I-Cl. swell, with EC(50) values of 2.6+/-0.1 and 2.8+/-0.4 mu mol/L, respect ively. The concentration-response curve for phenylephrine was shifted significantly (P<.001) to the right by the alpha(1)-adrenoceptor antag onist prazosin and by the alpha(1A)-receptor antagonists (+)-niguldipi ne and 5-methylurapidil but was unaltered by the alpha(1B)-receptor an tagonist chloroethylclonidine (100 mu mol/L). Inhibition of protein ki nase C (PKC) with staurosporine, H-7, or 18-hour preincubation with th e phorbol ester 4 beta-phorbol 12-myristate 13-acetate (PMA, 500 nmol/ L) blocked the effects of phenylephrine on I-Cl.swell, and the highly selective PKC inhibitor bisindolylmaleimide blocked the effects of nor epinephrine on I-Cl.swell and I-Cl.b. Both PMA and 1-oleoyl-2-acetylgl ycerol inhibited I-Cl.swell in a concentration-dependent fashion. In b linded studies, the phorbol ester phorbol 12,13-didecanoate (PDD) redu ced I-Cl.swell by 91+/-3%; its inactive analogue 4 alpha-PDD had no ef fect (mean change, 3+/-1%). Preincubation with pertussis toxin (PTX) p revented the actions of phenylephrine on I-Cl.swell, indicating a role for a PTX-sensitive guanine nucleotide-binding (G) protein. We conclu de that alpha-adrenergic agonists inhibit volume-sensitive Cl- current s in rabbit atrial cells by interacting with an alpha(1A)-adrenoceptor mechanism that is coupled to PKC via a PTX-sensitive G protein. These results suggest a potentially novel mechanism of alpha-adrenergic con trol of cardiac electrical activity, the inhibition of volume-sensitiv e Cl- currents, and indicate that PKC, well known to elicit phosphoryl ation-dependent Cl- currents in cat and guinea pig ventricular myocyte s, is also capable of potently inhibiting other forms of cardiac Cl- c urrent.