Physostigmine-induced contractures in frog skeletal muscle |
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Authors: | Pagala Murali Krishna Dass Sandow Alexander |
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Affiliation: | (1) Department of Biology, New York University, 10003 New York, New York, USA;(2) Present address: Neuromuscular Disease Division, Maimonides Medical Center, 4802 Tenth Ave, 11219 Brooklyn, N. Y., USA |
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Abstract: | Summary Physostigmine in 15 mM concentration at pH 8.4 produces reversible contractures of up to 0.3 P0 tension output in frog's whole toe muscle or in 7–10 fiber bundles of these muscles. At pH 7.2, the 15 mM physostigmine contracture output is only about 0.10 P0. The 15 mM, pH 8.4 contractures are essentially unaffected by lack of external Ca2+, complete depolarization of the fibers, detubulation by glycerol treatment, and 0°C ambient temperature. These results and other evidence indicate that physostigmine produces contracture by directly releasing activator Ca2+ from the sarcoplasmic reticulum (SR). Pretreatment of muscles with 4 mM procaine reduces physostigmine's capacity to produce contracture, evidently by means of a competitive inhibition at SR sites. The above results indicate similarities between physostigmine and caffeine contractures. But the physostigmine action differs in that it is reversible, and, especially, it lacks the ability, strongly characteristic of caffeine, to sensitize a muscle to produce a rapid cooling contracture. The internal action of physostigmine requires that it be permeant, and since it is a weak base (pKa=8.2), this property is provided by its uncharged base. But, once internal, where the pH =6.8, most of the drug will be protonated and it may act on the SR in this form, in contrast with caffeine which, since its pKa is about 1.0, acts on the SR as uncharged base. |
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Keywords: | Physostigmine contracture Sarcoplasmic reticulum Rapid cooling contracture Procaine inhibition of physostigmine contracture T-tubule disruption |
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