Enhanced Ca2+-induced calcium release by isolated sarcoplasmic reticulum vesicles from malignant hyperthermia susceptible pig muscle.

To further define the possible involvement of sarcoplasmic reticulum calcium accumulation and release in the skeletal muscle disorder malignant hyperthermia (MH), we have examined various properties of sarcoplasmic reticulum fractions isolated from normal and MH-susceptible pig muscle. A sarcoplasmic reticulum preparation enriched in vesicles derived from the terminal cisternae, was further fractionated on discontinuous sucrose density gradients (Meissner, G. (1984) J. Biol. Chem. 259, 2365-2374). The resultant MH-susceptible and normal sarcoplasmic reticulum fractions, designated F0-F4, did not differ in yield, cholesterol and phospholipid content, or nitrendipine binding capacity. Calcium accumulation (0.27 mumol Ca/mg per min at 22 degrees C), Ca2+-ATPase activity (0.98 mumol Pi/mg per min at 22 degrees C), and calsequestrin content were also similar for MH-susceptible and normal sarcoplasmic reticulum fraction F3. To examine sarcoplasmic reticulum calcium release, fraction F3 vesicles were passively loaded with 45Ca (approx. 40 nmol Ca/mg), and rapidly diluted into a medium of defined Ca2+ concentration. Upon dilution into 1 microM Ca2+, the extent of Ca2+-dependent calcium release measured after 5 s was significantly greater for MH-susceptible than for normal sarcoplasmic reticulum, 65.9 +/- 2.8% vs. 47.7 +/- 3.9% of the loaded calcium, respectively. The C1/2 for Ca2+ stimulation of this calcium release (5 s value) from MH-susceptible sarcoplasmic reticulum also appeared to be shifted towards a higher Ca2+-sensitivity when compared to normal sarcoplasmic reticulum. Dantrolene had no effect on calcium release from fraction F3, however, halothane (0.1-0.5 mM) increased the extent of calcium release (5 s) similarly in both MH-susceptible and normal sarcoplasmic reticulum. Furthermore, Mg2+ was less effective at inhibiting, while ATP and caffeine were more effective in stimulating, this Ca2+-dependent release of calcium from MH-susceptible, when compared to normal sarcoplasmic reticulum. Our results demonstrate that while sarcoplasmic reticulum calcium-accumulation appears unaffected in MH, aspect(s) of the sarcoplasmic reticulum Ca2+-induced calcium release mechanism are altered. Although the role of the Ca2+-induced calcium release mechanism of sarcoplasmic reticulum in situ is not yet clear, our results suggest that an abnormality in the regulation of sarcoplasmic reticulum calcium release may play an important role in the MH syndrome.

Effects of monensin and forage:concentrate ratio on feed intake, endocrine, and ovarian function in beef heifers.

The objective of this study was to determine the effects of monensin (M) and altered forage:concentrate ratio (F:C) on feed intake, hormonal profiles, and ovarian function in beef heifers. Four ruminally cannulated heifers were maintained in metabolism crates and fed twice daily. Diets contained 0 or 200mg per day M in diets of 70:30 (high forage, HF) or 30:70 (high concentrate, HC) F:C. Diets were limit fed to achieve isocaloric intake of calculated ME. Monensin supplementation had no effect on feed intake, but heifers fed HF had greater refusals compared with heifers fed HC (P<0.005). Serum insulin, progesterone, and estradiol concentrations did not differ between diets or treatments (P>0.10). Diet and/or treatment tended to alter the size or number of follicles on either or both ovaries. Monensin resulted in more waves of follicular development per estrous cycle with HF diets, but fewer with HC diets (P<0.07). The interovulatory interval, lifespan of the corpus luteum, and the size of the dominant follicle in the first two follicular waves were not different due to diet or treatment. However, the HC diet tended to produce a larger ovulatory follicle compared with the HF diet (HF=12.25, HC=13.5; P<0.08). We conclude that feeding monensin affects estradiol secretion and a higher proportion of concentrates alters ovarian dynamics, which could affect fertility in beef heifers.

Effect of forage:concentrate ratio on digestion and reproduction in primiparous beef heifers.

We evaluated the effects of high- (HF) and moderate- (MF) forage diets on digestive and reproductive characteristics in beef heifers. Thirty primiparous beef heifers were allotted by weight and backfat thickness to receive either 80:20 (HF) or 50: 50 (MF) forage:concentrate ratio diets from parturition to at least 90 d postpartum. Alfalfa hay and wheat straw were the forage sources and barley was the concentrate source. Equal daily amounts of ME were provided to all heifers by restricting intake of the MF diet. Digestibility of DM was greater (P < .001) for MF compared with HF diets, whereas NDF digestibility was not different. Dry matter and NDF digested daily was lower (P < .001) for MF than for HF diets. Ruminal fluid pH was lower (P < .05) for MF diets; however, the acetate:propionate ratio was not different. Serum insulin concentrations were greater for MF diets for all hours (P < .001) and weeks (P < .05) of sampling. Changes in weight, backfat thickness, and body condition score at 90 d postpartum were not different between treatments. Calf gain to 30 d, however, was greater (P < .10) for the MF than for the HF treatment (25.5 vs 20.7 kg). Maximum size of the ovulatory follicle was greater (P < .10) for cows receiving the HF diet than for cows receiving the MF diet. However, other aspects of ovarian follicular growth and wave dynamics and the intervals from parturition to first and second ovulation, first estrus, first service, and conception were not different between treatments. Shifts in energy supply from forage to concentrate had minimal effect on digestion and reproduction in first-calf beef heifers in this study.

Malignant hyperthermia susceptibility: cardiac histomorphometry of dogs and young and market-weight swine.

The defect causing malignant hyperthermia has been proposed to involve cardiac as well as skeletal muscle. We tested the hypothesis that histomorphometric parameters for ventricular wall from malignant hyperthermia-susceptible swine and dogs were abnormal. Hearts were obtained from: mature dogs, age- and weight-matched young swine (89 +/- 15 days, 30 +/- 3 kg); and market-weight swine (102 +/- 10 kg). Using light microscopy, estimates were made for muscle nuclear dimensions and the volume-fraction of nuclei, sarcoplasm, blood vessels, and interstitial space. Cardiac maturation in both MH and normal swine was accompanied by decreased myocyte volume-fraction due to decreased nuclear volume-fraction and increased interstitial space volume-fraction. Sarcoplasm and vasculature volume-fraction were unchanged after maturation. Nuclear volume-fraction was slightly greater (p less than 0.05) in the right ventricle than the left for malignant hyperthermia and normal swine. Myocyte nuclear dimensions were generally similar among animals. Dogs and the oldest group of swine were not significantly different. Myocytes of all swine contained multiple nuclei, closely spaced in rows of 2 to 12. In contrast, most myocytes of mature dogs apparently contained one or two nuclei. Histomorphometric values were not significantly different between normal and malignant hyperthermia young swine and dogs. However, within the market-weight swine, volume-fraction for malignant hyperthermia myocytes and myocyte nuclei was decreased and interstitial space was increased compared to normal.(ABSTRACT TRUNCATED AT 250 WORDS)