RyR3 amplifies RyR1-mediated Ca(2+)-induced Ca(2+) release in neonatal mammalian skeletal muscle.

The neonatal mammalian skeletal muscle contains both type 1 and type 3 ryanodine receptors (RyR1 and RyR3) located in the sarcoplasmic reticulum membrane. An allosteric interaction between RyR1 and dihydropyridine receptors located in the plasma membrane mediates voltage-induced Ca(2+) release (VICR) from the sarcoplasmic reticulum. RyR3, which disappears in adult muscle, is not involved in VICR, and the role of the transiently expressed RyR3 remains elusive. Here we demonstrate that RyR1 participates in both VICR and Ca(2+)-induced Ca(2+) release (CICR) and that RyR3 amplifies RyR1-mediated CICR in neonatal skeletal muscle. Confocal measurements of intracellular Ca(2+) in primary cultured mouse skeletal myotubes reveal active sites of Ca(2+) release caused by peripheral coupling between dihydropyridine receptors and RyR1. In myotubes lacking RyR3, the peripheral VICR component is unaffected, and RyR1s alone are able to support inward CICR propagation in most cells at an average speed of approximately 190 microm/s. With the co-presence of RyR1 and RyR3 in wild-type cells, unmitigated radial CICR propagates at 2,440 microm/s. Because neonatal skeletal muscle lacks a well developed transverse tubule system, the RyR3 reinforcement of CICR seems to ensure a robust, uniform, and synchronous activation of Ca(2+) release throughout the cell body. Such functional interplay between RyR1 and RyR3 can serve important roles in Ca(2+) signaling of cell differentiation and muscle contraction.

Increased susceptibility to fatigue of slow- and fast-twitch muscles from mice lacking the MG29 gene.

Mitsugumin 29 (MG29), a major protein component of the triad junction in skeletal muscle, has been identified to play roles in the formation of precise junctional membrane structures important for efficient signal conversion in excitation-contraction (E-C) coupling. We carried out several experiments to not only study the role of MG29 in normal muscle contraction but also to determine its role in muscle fatigue. We compared the in vitro contractile properties of three muscles types, extensor digitorum longus (EDL) (fast-twitch muscle), soleus (SOL) (slow-twitch muscle), and diaphragm (DPH) (mixed-fiber muscle), isolated from mice lacking the MG29 gene and wild-type mice prior to and after fatigue. Our results indicate that the mutant EDL and SOL muscles, but not DPH, are more susceptible to fatigue than the wild-type muscles. The mutant muscles not only fatigued to a greater extent but also recovered significantly less than the wild-type muscles. Following fatigue, the mutant EDL and SOL muscles produced lower twitch forces than the wild-type muscles; in addition, fatiguing produced a downward shift in the force-frequency relationship in the mutant mice compared with the wild-type controls. Our results indicate that fatiguing affects the E-C components of the mutant EDL and SOL muscles, and the effect of fatigue in these mutant muscles could be primarily due to an alteration in the intracellular Ca homeostasis.

Acute cardiotoxicity of moniliformin in broiler chickens as measured by electrocardiography.

Electrocardiography was used to examine the acute cardiotoxic effects of moniliformin on 3-week-old broiler chickens. Each of the seven pairs of anesthetized birds (pentobarbital sodium, 40 mg/kg body weight, intramuscular) was injected intravenously with moniliformin (1 mg/kg body weight) or an equal volume of normal saline (1 ml/kg body weight), and changes in electrocardiogram were monitored for 50 minutes. Three of the seven birds injected with moniliformin died within 50 minutes post-injection. Moniliformin caused a bradycardia, which became highly significant (P < 0.05) within 15 minutes post-injection. The P-R, Q-T, and S-T intervals of moniliformin-injected birds were significantly lengthened throughout the 50-minute observation (P < 0.05). The results indicate that the moniliformin-induced mortality is due primarily to cardiac failure.

Toxicity of corn culture material of Fusarium proliferatum M-7176 and nutritional intervention in chicks.

The toxicity of Fusarium proliferatum M-7176 cultured on corn (FPC) and nutritional intervention were investigated in baby chicks (New Hampshire x Single Comb White Leghorn) in three 2-wk feeding experiments. In Experiment 1, 30% FPC decreased weight gain (P < .05) and increased relative heart weight (RHW) (P < .01). Experiment 2 included a 2 x 2 factorial arrangement of FPC (0 or 30%) and Se (0 or 5 mg/kg) and two detached treatments of Se (2.5 mg/kg) or thiamin (B1, 25 mg/kg) supplementations to 30% FPC. Only B1 was inhibitory to the toxic effects of FPC on weight gain, feed efficiency, and RHW (P < .05). Experiment 3 included 2 x 2 factorial arrangement between FPC (0 or 30%) and Se (0 or 4 mg/kg), or B1 (0 or 50 mg/kg), or vitamin E (0 or 50 IU/kg) and additional supplementations of Se (2 mg/kg), B1 (10 or 25 mg/kg), or E (10 IU/kg) to 30% FPC. A new batch of FPC was used and it caused 36% mortality. Vitamin E did not interact with FPC, but SE interacted with FPC only on RHW (P < .01). Thiamin interacted with FPC on all measured variables with significance ranging from P < .1 to P < .01. Supplementation of B1 as low as 10 mg/kg was inhibitory to some toxic effects of FPC. However, B1 as high as 50 mg/kg did not completely negate the cardiotoxicity. Water-extractable B1 in FPC diets was 13 to 27% of the control diets. Water extract of FPC reduced B1 recovery from a standard solution by 40%. The anti-thiamin factor was heat-sensitive. Both fumonisins and moniliformin were present in FPC. However, the results indicate that the anti-thiamin factor is also a major toxic factor of F. proliferatum M-7176.