Skeletal muscle junctional membrane protein content in pigs with different ryanodine receptor genotypes

The content of the sarcoplasmic reticulum (SR) Ca²⁺-ATPase, transverse tubule dihydropyridine receptor (DHPR), and SR ryanodine receptor (RyR) was determined in muscle of pigs homozygous for the normal RyR allele and homozygous or heterozygous for the malignant hyperthermia-susceptible (MHS) RyR allele. Total muscle membranes isolated from 1-day-old pigs of the three different genotypes did not differ in the content of any of these proteins. However, at 28 days of age, crude membranes and total muscle homogenates from homozygous MHS pigs exhibited only 61-81% of the [³H]PN 200-110 or [³H]ryanodine binding of identical preparations isolated from normal pigs; these MHS membranes also contained only 50% of the normal content of each of the DHPR subunits. The crude membranes and muscle homogenates from heterozygous pigs were intermediate to both types of homozygotes in terms of [³H]PN 200-110 binding, [³H]ryanodine binding, and the content of the DHPR subunits. However, membrane preparations enriched in tri-adic junctional proteins isolated from 3- to 4-mo-old pigs of the three different genotypes did not differ in their [³H]PN 200-110 binding, [³H]ryanodine binding, or Ca²⁺-ATPase activities. We conclude that, although the stoichiometry of the RyR to DHPR is not altered, the presence of the MHS RyR allele during muscle development results in a decreased relative content of these two proteins. This is probably due to a lower junctional membrane content and may be an important ultrastructural consequence of the altered sarcoplasmic Ca²⁺ regulation in MHS muscle.