Abnormal ryanodine receptor channels in malignant hyperthermia

Abstract

Previous studies have demonstrated a defect associated with the calcium release mechanism of sarcoplasmic reticulum (SR) from individuals susceptible to malignant hyperthermia (MH). To examine whether SR calcium release channels were indeed altered in MH, SR vesicles were purified from normal and MH susceptible (MHS) porcine muscle. The Ca2+ dependence of calcium efflux rates from 45Ca2(+)-filled SR vesicles was then compared with the Ca2+ dependence of single-channel recordings of SR vesicles incorporated into planar lipid bilayers. The rate constants of 45Ca2+ efflux from MHS SR were two to threefold larger than from normal SR over a wide range of myoplasmic Ca2+. Normal and MHS single channels were progressively activated in a similar fashion by cis Ca2+ from pCa 7 to 4. However, below pCa 4, normal channels were inactivated by cis Ca2+, whereas MHS channels remained open for significantly longer times. The altered Ca2+ dependence of channel inactivation in MHS SR was also evident when Ca2+ was increased on the trans side while cis Ca2+ was held constant. We propose that a defect in a low-affinity Ca2+ binding site is responsible for the altered gating of MHS SR channels. Such a defect could logically result from a mutation in the gene encoding the calcium release channel, providing a testable hypothesis for the molecular basis of this inherited disorder.

Calcium dependence of ryanodine‐sensitive calcium channels from brain cortex endoplasmic reticulum

“…However, lower calcium affinity, as evidenced by the 6-fold higher K0.5 values for activation, was observed in this case. The response to cis Ca of the calcium stimulated and calcium inhibited channels of brain cortex microsomes is similar to that found previously for the calcium channels of mammalian skeletal SR [21,26], which correspond to the ryanodine receptor-I isoform [23]. A similar effect of Ca was also found in one of the two channel types present in fish skeletal muscle [24], and in half of the channels present in SR from frog skeletal muscle [18].…”

Inhibition of the skeletal muscle ryanodine receptor calcium release channel by nitric oxide

“…The above results obtained from flux measurements suggest that NO might inhibit the RyRC, the principal Ca 2+ release pathway in striated muscles. To directly test this possibility, sucrose gradient purified SR vesicles were fused into planar lipid bilayers [15,19] and unitary Cs ÷ currents of single RyRCs [16] were studied in the presence and the absence of NO donors. The single channels in the bilayer were identified by both their conductance (408 + 12 pS) and their typical response [19] to ruthenium red (not shown) as well as to ryanodine (Fig.…”

“…CsCH3SO3 solutions (i.e. Cs + as current carrier) buffered to pH 7.4 with 10 mM MOPS (250 mM cis, 50 mM trans) were used to isolate the channel from other ionic conductances and to improve signal to noise ratio as described [16]. Single channel conductance was determined by measuring the current amplitudes at 0, +20 and +30 mV holding potential of fully resolved openings.…”

Sarcoplasmic reticulum release channels from frog skeletal muscle display two types of calcium dependence

“…Calcium channels present in isolated vesicles from SR of rabbit [9-111, frog [12,13], pig [14] and human [15] skeletal muscle have been studied after fusion with planar lipid bilayers. These channels are activated or blocked by the same agents that modify calcium release from SR vesicles.…”