Phenytoin alters transcript levels of hormone-sensitive lipase in muscle from horses with hyperkalemic periodic paralysis

In equine hyperkalemic periodic paralysis (HyperPP), there is evidence suggesting that the primary defect in the sodium channel is associated with a secondary alteration in triacylglycerol-associated fatty acid metabolism (TAFAM) in skeletal muscle. Furthermore, TAFAM may be involved in the therapeutic action of phenytoin. The effects of phenytoin treatment on the transcript levels of three key proteins in TAFAM, hormone sensitive lipase (HSL), carnitine palmitoyltransferase (CPT), and fatty acid binding protein (FABP), were examined. These transcripts were quantitated by competitive reverse transcription polymerase chain reaction in undifferentiated and differentiated primary cultures of equine skeletal muscle from control, heterozygous HyperPP, and homozygous-affected HyperPP horses. There was a 10-fold lower level of HSL transcript in both undifferentiated and differentiated cultures from homozygous-affected horses than from horses of the other genotypes. Phenytoin selectively increased the HSL transcript in homozygous-affected differentiated cultures to levels similar to those of the other genotypes. The levels of CPT and FABP transcripts were unaffected by genotype, differentiation, and phenytoin treatment. These results suggest that the primary defect in HyperPP may secondarily decrease HSL transcript levels and that the therapeutic action of phenytoin may include regulation of mRNA transcripts in skeletal muscle.

Myotoxin a reduces the threshold for calcium-induced calcium release in skeletal muscle

Myotoxin a, isolated from the venom of the prairie rattlesnake Crotalus viridis viridis, induces necrosis in skeletal muscle. In isolated organelles, it has been reported that myotoxin a reduces Ca2+ uptake into the sarcoplasmic reticulum. The effects of the toxin on Ca2+ regulation were examined in heavy sarcoplasmic reticulum fractions from human and equine skeletal muscle. Ca2+ uptake and release (the threshold of Ca(2+)-induced Ca2+ release) were examined by dual wavelength spectrophotometry. The toxin lowered the threshold of Ca(2+)-induced Ca2+ release in a dose-dependent manner (1-10 microM) and this effect was antagonized by ruthenium red, a Ca2+ release channel blocker. Ca2+ uptake into equine heavy sarcoplasmic reticulum was not decreased by myotoxin a (10 microM) when Ca2+ release was blocked by ruthenium red. [3H]Ryanodine binding to equine heavy sarcoplasmic reticulum was converted from a relatively low affinity state to a higher affinity state by myotoxin a. These results suggest that the dominant effect of myotoxin a is to increase the Ca2+ sensitivity for the opening of the calcium release channel (ryanodine receptor). Myotoxin a may prove to be a useful tool to probe the modulation of calcium release in sarcoplasmic reticulum fractions.

Effects of a cardiotoxin from Naja naja kaouthia venom on skeletal muscle: involvement of calcium-induced calcium release, sodium ion currents and phospholipases A2 and C

Snake venom cardiotoxin (CTX) fractions induce contractures of skeletal muscle and hemolysis of red blood cells. The fractions also contain trace amounts of venom-derived phospholipase A2 (PLA2) contamination and activate tissue phospholipase C (PLC) activity. The present study examines the mechanisms of action of a CTX fraction from Naja naja kaouthia venom in skeletal muscle. Sphingosine competitively antagonized CTX-induced red blood cell hemolysis, but not skeletal muscle contractures. CTX rapidly lowered the threshold for Ca(2+)-induced Ca2+ release in heavy sarcoplasmic reticulum fractions, as monitored with arsenazo III. There was also a slower time-dependent reduction of Na+ currents, as assessed by whole cell patch-clamp techniques. The CTX fractions elevated levels of free fatty acids and diacylglycerol for 2 hr in primary cultures of human skeletal muscle by a combined action of venom-derived PLA2 contamination in the fraction and activation of endogenous PLC activity. The activation of tissue PLC activity could be readily distinguished from the contribution of the venom PLA2 by p-bromophenacyl bromide treatment of CTX fractions. The mechanism of action involved in contractures of skeletal muscle appears to be related to the immediate and specific effect of CTX (Ca2+ release by the sarcoplasmic reticulum), while the mechanisms involved in hemolysis of red blood cells and decreased Na+ currents in skeletal muscle most likely relate to long-term effects on lipid metabolism.