Molecular Pharmacology, Vol 19, 411-424, Copyright © 1981 by the American Society for Pharmacology and Experimental Therapeutics

The Pharmacology of Pumiliotoxin-B

I. Interaction with Calcium Sites in the Sarcoplasmic Reticulum of Skeletal Muscle

¹ Department of Pharmacology and Experimental Therapeutics, School of Medicine, University of Maryland, Baltimore, Maryland 21201, and Laboratory of Bioorganic Chemistry, National Institute of Arthritis, Metabolism and Digestive Diseases, National Institutes of Health, Bethesda, Maryland 20205

The actions of pumiliotoxin-B (PTX-B), a novel indolizidine alkaloid from the skin of the Panamanian frog, Dendrobates pumilio, have been studied in skeletal muscle of rat, frog, and crayfish with electrophysiological and biochemical techniques. PTX-B reversibly potentiates and prolongs the direct elicited muscle twitch in rat and frog skeletal muscle up to 12-fold in a concentration- and frequency-dependent manner, the potentiation being greater at the lower frequencies of stimulation. Responses of the muscle to tetanic stimulation in the presence of PTX-B are potentiated more at 10 and 20 Hz than at 50 and 100 Hz; tetanic fusion occurs earlier, and an aftercontraction is present when tetanic stimulation occurs in the presence of PTX-B. The twitch/tetanus ratio at 100 Hz is increased in the presence of PTX-B from 0.3 to more than 1.1 as a result of the increase in twitch amplitude. These effects on frog skeletal muscle are seen in the absence of any effect of PTX-B on spontaneous and evoked transmitter release, acetylcholinesterase activity, muscle action potential, delayed rectification, and cable properties of the muscle fiber. In the absence of external calcium, PTX-B prolongs but does not potentiate the twitch, while methoxyverapamil and dantrolene only partially suppress the actions of PTX-B. In crayfish skeletal muscle, PTX-B increases the rate of rise of the "calcium-dependent" action potential and shortens its duration. Biochemical studies reveal that PTX-B inhibits calcium-dependent adenosine triphosphatase from sarcoplasmic reticulum preparations of both frog and rat skeletal muscles in a concentration- and calcium-dependent manner. We suggest that PTX-B potentiates and prolongs the muscle twitch by (a) facilitating the release of calcium from storage sites within the sarcoplasmic reticulum, (b) mobilizing calcium from extracellular sites, and (c) blocking the reuptake of calcium by calcium-dependent adenosine triphosphatase.

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ACKNOWLEDGMENT We are indebted to Ms. Mabel Alice Zelle for expert technical assistance.