Electrophysiological effects of alpha 2-adrenergic stimulation in canine cardiac Purkinje fibers.
The effects of alpha 2-adrenergic stimulation on action potentials were measured in isolated canine Purkinje fibers. Action potential durations at 50 and 90% of repolarization (APD50 and APD90) were significantly prolonged by 0.25 microM l-norepinephrine + 0.5 microM dl-propranolol (NE+P) from baseline values of 166 +/- 7 and 249 +/- 9 (SE) ms (n = 7) to 174 +/- 7 and 265 +/- 9 ms, respectively (P < 0.05 for both). Selective alpha 2-blockade with 0.01 microM yohimbine (YO) reduced this prolongation by NE+P in APD50 and APD90 to 169 +/- 7 and 256 +/- 8 ms, respectively (P < 0.05 compared with NE+P). Additional selective alpha 1-blockade with 0.01 microM prazosin (PZ) completely blocked the effects of NE+P, returning APD50 and APD90 to 163 +/- 7 and 250 +/- 9 ms (not different from baseline). After incubation of isolated Purkinje fibers with pertussis toxin (1 microgram/ml), which reduced the availability of a 41-kDa membrane protein for ADP ribosylation by 70 +/- 7% (n = 4, P < 0.05), YO failed to reverse the prolongation in action potential durations brought on by NE+P, but the effects of PZ were intact. The effects of alpha 2-stimulation on beta-adrenergic-induced delayed afterdepolarizations (DADs) were studied by burst pacing of Purkinje fibers in Tyrode solution containing 7.5 mM Ca2+. The DADs induced in the presence of NE+PZ (beta- + alpha 2-stimulation) were significantly smaller in amplitude and required a shorter pacing cycle length to reach threshold than those induced in the presence of NE+PZ+YO (unopposed beta-adrenergic stimulation). Furthermore sustained triggered activity, seen in five of eight preparations under beta-stimulation, could no longer be elicited in the presence of beta- + alpha 2-stimulation. These results suggest that the postjunctional alpha 2-adrenergic receptors in canine Purkinje fibers are coupled to a pertussis toxin-sensitive G protein and that stimulation of these receptors leads to action potential prolongation and suppression of DADs induced by beta-adrenergic stimulation.
Samson
R A
RA
Department of Pediatrics, College of Medicine, University of Iowa, Iowa City 52242, USA.
Cai
J J
JJ
Shibata
E F
EF
Martins
J B
JB
Lee
H C
HC
D000200:Action Potentials / Q000187:drug effects; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000818:Animals; D000068438:Brimonidine Tartrate; D004285:Dogs; D004594:Electrophysiology; D005260:Female; D008297:Male; D037342:Pertussis Toxin; D011690:Purkinje Fibers / Q000187:drug effects* / Q000502:physiology; D011810:Quinoxalines / Q000494:pharmacology; D011943:Receptors, Adrenergic, beta / Q000187:drug effects / Q000502:physiology; D016276:Ventricular Function / Q000187:drug effects; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
D000316:Adrenergic alpha-Agonists; D011810:Quinoxalines; D011943:Receptors, Adrenergic, beta; D010566:Virulence Factors, Bordetella; D000068438:Brimonidine Tartrate; D037342:Pertussis Toxin