ISSN:
1423-0127
Keywords:
Purkinje fiber
;
Muscarinic receptor
;
Na+, cellular
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Medicine
Notes:
Abstract In this study, the effects of carbachol (CCh) on twitch tension, intracellular Na+ activity (a Na i ), and action potential were simultaneously measured in canine cardiac Purkinje fibers in order to examine the regulation of inotropy through muscarinic receptors and its relation to a Na i . In fibers driven at 1 Hz, CCh (10 µM) initially and transiently decreased and then increased the twitch tension by 36±8%. The action potential showed a significant elevation of the plateau and a significant shortening of the duration at 90% repolarization (APD90), from 403±7 to 389±7 ms. The a Na i decreased from 7.4±0.4 to 6.7±0.3 mM (n=23, p〈0.05). Atropine (1 µM) decreased the twitch tension by 21±6% (n=7, p〈0.05) without significant effects on the action potential and a Na i , and inhibited the effects of CCh. Cs+ (20 mM) increased the plateau height and APD90, enhanced the twitch tension by 66±24%, but decreased a Na i from 7.3±0.3 to 6.3±0.4 mM (n=6, p〈0.05). In the presence of 20 mM Cs+, some fibers generated slow responses. The addition of 10 µM CCh further increased the twitch tension and APD90, and decreased a Na i from 6.3±0.4 to 5.3±0.3 mM. Ouabain (0.3 µM) increased the twitch tension and a Na i , and inhibited the CCh-induced decrease of a Na i . In the presence of ouabain, 20 mM Cs+ depolarized the fiber and generated slow responses with a decreased a Na i . The addition of 10 µM CCh enhanced the slow action potential, and increased a Na i although there was a transient decrease during early exposure. These results suggest that activation of muscarinic receptors in canine Purkinje fibers results in an enhancement of the Na+-K+ pump activity and a biphasic inotropic response, probably via different receptor subtypes. The inhibitory effect, most likely through M2 receptors, is associated with the activation of K+ channels. The stimulatory effect, on the other hand, is probably due to the action on the M1 receptors, resulting in increases in Ca2+ currents.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF02255901
Permalink
