Key result
Isoproterenol (1 mumol/L) increased squared conduction velocity from 3.46 to 3.85 (m/s)2 at physiological membrane potentials (P<0.011), but decreased it at depolarized potentials.
Absolute Event Rate: 3.85% vs 3.46%
p-value: p=<0.011
Beta-adrenergic stimulation with isoproterenol exerts a voltage-dependent modulation on conduction in canine Purkinje fibers, enhancing it at normal resting potentials but depressing it at depolarized potentials.
Voltage-dependent beta-adrenergic effects on conduction merit caution in depolarized states; leaves open in vivo translation.
Although recent voltage-clamp and microelectrode studies have demonstrated beta-adrenergic modulation of Na+ current (INa) the modulation of conduction by catecholamines and the voltage dependence of that process have not been elucidated. To determine whether voltage-dependent modulation of conduction occurs in the presence of a beta-adrenergic agonist, the effect of 1 mumol/L isoproterenol on impulse propagation in canine Purkinje fibers was examined by using a dual-microelectrode technique. At physiological membrane potentials ([K+]o 5.4 mmol/L), isoproterenol increased squared conduction velocity [theta 2, 0.39 +/- 0.25 (m/s)2 (mean +/- SD)] from 3.46 +/- 0.86 to 3.85 +/- 0.98 (m/s)2 (P < .011), an 11% change, without altering the maximum first derivative of the upslope of phase 0 of the action potential (Vmax, 641 +/- 50 versus 657 +/- 47 V/s, P = NS). At transmembrane potential of -65 mV, produced by 12 mmol/L [K+]o titration, theta 2 declined 79% to 0.73 +/- 0.44 (m/s)2 as Vmax decreased 85% to 95 +/- 43 V/s (P < .02). The addition of isoproterenol further decreased theta 2 to 0.49 +/- 0.33 (m/s)2 (P = .02) in parallel with a further decline in Vmax to 51 +/- 25 V/s (P < .05). Isoproterenol produced a 3-mV hyperpolarizing shift of apparent Na+ channel availability curves generated from both theta 2 and Vmax, used as indexes of the fast inward INa, without changing the slopes of the relation. The relation between normalized theta 2 and Vmax over a range of depolarized potentials was linear and was not appreciably altered by isoproterenol.(ABSTRACT TRUNCATED AT 250 WORDS)
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Munger et al. (1994) studied Canine Purkinje fibers (experimental model). Isoproterenol vs. Baseline/Control was evaluated on Squared conduction velocity [theta 2, (m/s)2] at physiological membrane potentials (p=<0.011). Isoproterenol (1 mumol/L) increased squared conduction velocity from 3.46 to 3.85 (m/s)2 at physiological membrane potentials (P<0.011), but decreased it at depolarized potentials.
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