James L. Poland

The rate and capacity of calcium uptake by sarcoplasmic reticulum in fast, slow, and cardiac muscle: Effects of ryanodine and ruthenium red

The rate and capacity of oxalate-supported calcium uptake was measured in homogenates of rat fast, slow, and cardiac muscle. The contribution of the releasing fraction of the sarcoplasmic reticulum (SR) to the calcium uptake abilities was estimated using ruthenium red or ryanodine to block the release channel. A relatively small fraction (12-20%) of the calcium pumping activity was associated with the release channel in skeletal muscle compared to 50% or more in cardiac muscle. The total capacity of the SR in the muscle types was in the ratio 1:0.75:1.5 for cardiac, slow, and fast muscle, respectively, while the rates of uptake were in the ratio 1:3.8:14.4. The major difference in the muscle types appears to be the density of pumping activity in the SR rather than the volume of the SR. The difference in the density of pumping activity is due to intrinsic differences in the kinetics of the calcium pump units and in their surface density.

Contribution of the ryanodine-sensitive fraction to capabilities of cardiac SR.

The contribution of the ryanodine-sensitive fraction of canine cardiac sarcoplasmic reticulum to the total issue calcium uptake was estimated by the oxalate-supported calcium uptake rate in canine whole heart homogenates. Ryanodine stimulated this uptake rate nearly three-fold. Ryanodine stimulated this same activity in isolated SR vesicles only two-fold. An analysis of the yield of calcium uptake activity throughout the isolation procedure showed that the largest discrimination between ryanodine-sensitive and ryanodine-insensitive activity occurs at the first centrifugation step. In isolated vesicles, the initial rate of uptake in the absence of oxalate correlated well with the sustained oxalate-support calcium uptake rate, suggesting that oxalate-supported calcium uptake is a good indicator of in vivo SR function. The similarity between the effects of ryanodine on the rate and capacity of calcium uptake in the presence or absence of oxalate is consistent with earlier observations that ryanodine adds effective volume by closing a calcium release channel in a subpopulation of SR. The data also suggest that the ratio of calcium pumping activity to volume in these two populations is not greatly different.

Mechanical and subcellular function of rat myocardium during chronic ethanol consumption.

Abstract Hemodynamic, mechanical, and subcellular function in a rat model of chronic ethanol consumption (A) and pair-fed control rats (C) were evaluated over a 17-week course. Between 5 and 17 weeks, isovolumetric peak systolic pressure did not change (C = 260 ± 20 mmHg; A = 265 ± 35 mmHg;p > 0.1). At 17 weeks myocardial contractility as evaluated by (dP/dt-max)/32 x LVP began to decline in the A rats (Vmax = 1.46 ± 0.12, r = 0.89; C = 1.560 ± 0.09, r = 0.95) but this is not statistically significant. Neither the velocity of calcium uptake by the isolated sarcoplasmic reticulum (SR) nor the whole heart homogenate (H) was different between C and A rats during the 17-week period and the estimated quantitative content of SR/g Heart did not change (at 17 weeks: mg SR/g Heart = 0.15, C and A). Utilizing Triton X-100 purification of cardiac myofibrils, Mg2+-dependent, Ca2+-stimulated myofribrillar ATPase activity began to decline at 11 weeks (A = 80% of control) and continued to decline at 17 weeks (A = 72% of control). These data suggest that a depression of contractile protein function may be an early alcohol-induced derangement in myocardial contractility that precedes the onset of significant mechanical dysfunction.

Myocardial Glycogen Changes with Exercise

Summary The extent of glycogenesis during exercise and the pattern of glycogen recovery following exercise is similar in trained and untrained rats. However, the maximum value reached during recovery is significantly higher in the trained group. A supercompensation of skeletal muscle glycogen was not observed during this same period. The degree of glycogenesis during a fast is the same in trained and untrained rats and in both groups is enhanced if the fast is begun following a bout of exercise.

Calcium uptake by sarcoplasmic reticulum of muscle grafts in rats

Cross transplantations were carried out in which the soleus (SOL) and extensor digitorum longus (EDL) muscles were switched to each others muscle bed. Sixty days later, oxalate-supported calcium uptake was measured in homogenates of these grafts and compared with calcium uptake by homogenates of the contralateral control EDL and SOL muscles. With the incubation conditions used, calcium uptake was essentially limited to sarcoplasmic reticulum (SR) vesicles. The velocities of the initial rapid calcium uptake were compared in the grafts and control muscles. Subsequently calcium uptake slowed and the 30-min accumulation of calcium indicated the loading capacity of the SR. In control muscles, the EDL had a faster velocity (0.234 +/- 0.011 mumol/mg/min) of calcium uptake and higher capacity (0.527 +/- 0.017 mumol/mg) for calcium loading than the SOL (0.089 +/- 0.008 mumol/mg/min and 0.26 +/- 0.014 mumol/mg, respectively). The EDL grafts (originally SOL muscles) had faster calcium uptakes than the control SOL muscles or SOL grafts (0.196 +/- 0.013 versus 0.089 +/- 0.008 or 0.126 +/- 0.024 mumol/mg/min). Also, the calcium uptake capacities were higher in EDL grafts than in control SOL muscles (0.400 +/- 0.017 versus 0.261 +/- 0.014 mumol/mg), but not statistically higher than in SOL grafts (0.360 +/- 0.033 mumol/mg). In contrast, SOL grafts (originally EDL muscles) had slower calcium uptakes (0.126 +/- 0.024 mumol/mg/min) than did the control EDL muscles or EDL grafts and the calcium uptake capacities (0.360 +/- 0.033 mumol/mg) were lower in SOL grafts than in control EDL muscles, but not statistically lower than in EDL grafts.(ABSTRACT TRUNCATED AT 250 WORDS)

The Effects of Prior Exercise on Myocardial Glycogenosis During a Fast

Summary The enhanced myocardial glycogenesis observed in trained rats can no longer be attributed to the training process but appears to be due merely to the last bout of exercise in the training program. The pattern of changes in cardiac glycogen during and following exercise involves more than simply depletion and recovery. It involves an additional period of relatively long duration during which the glycogen is significantly greater than control levels.

Calcium uptake by sarcoplasmic reticulum from nerve-intact and standard skeletal muscle grafts.

A freely grafted rat soleus muscle exhibits a decrease in velocity and capacity of SR calcium uptake. This deficit is not prevented by maintaining neural connections (nerve-intact graft) during grafting. Thus the greater mechanical capability of nerve-intact grafts, relative to standard grafts, is not accompanied by any enhancement of the SR tubules.