Biomedical subjects

G K Grimditch

Publications and source records attributed to G K Grimditch.

11 recordsLinked to original sources

PubMed1989

Exercise and insulin stimulate skeletal muscle glucose transport through different mechanisms.

This study was designed to examine the effects of acute exercise, insulin stimulation, and their combination on the kinetics of glucose transport in rat skeletal muscle. Sarcolemmal (SL) membranes were isolated from control (C), acute exercise (E), insulin-stimulated (I), and combined (E + I) rats. Michaelis-Menten kinetics indicated that the Vmax for glucose transport was increased after each perturbation compared with C but were not different from each other (E, 4,334 +/- 377; I, 4,424 +/- 668; E + I, 4,338 +/- 602; and C, 1,366 +/- 124 pmol.mg protein-1.s-1). The apparent Km was unchanged. Scatchard plots of cytochalasin B binding sites indicated that both I and E + I increased the number of binding sites compared both E and C (9.4 +/- 0.5 and 7.8 +/- 0.5 vs. 5.1 +/- 0.2 and 5.5 +/- 0.3 pmol/mg protein) without altering the dissociation constant. The increase in Vmax was greater than the increase in cytochalasin B binding sites indicating that both I and E + I caused an increase in the turnover rate of transport molecules as well as an increase in the total number of transport molecules. Because there was no change in the Km for glucose transport and no increase in cytochalasin B binding sites after exercise, the increase in Vmax was due solely to an increased turnover rate of existing transport molecules.

Animals↗

PubMed1989

Beta-adrenergic receptors are not responsible for exercise stimulation of glucose transport.

This study was designed to examine whether the increased glucose transport after acute exercise in rat skeletal muscle is mediated via beta-adrenergic receptor stimulation. Sarcolemmal (SL) membranes were isolated from three groups: control (C), acute exercise (E), and exercise + propranolol (E+P). The acute exercise bout was performed on a treadmill and consisted of a 45-min run until near exhaustion. E+P received an intravenous propranolol injection (0.8 mg/kg) 10 min before the exercise session. Michaelis-Menten kinetics at 1.5 s indicated that the Vmax for glucose transport was increased after each perturbation compared with C but were not different from each other (E, 4,334 +/- 377; E+P, 4,824 +/- 357; and C, 1,366 +/- 124 pmol.mg protein-1.s-1). The apparent Km's were similar in all groups. Scatchard plots for the D-glucose inhibitable class of cytochalasin B binding sites indicated no differences in either the total number of binding sites in the SL vesicles (C, 5.5 +/- 0.3; E, 5.1 +/- 0.2, and E+P, 5.1 +/- 0.3 pmol/mg protein) or in their dissociation constant (Kd) (C, 46 +/- 3; E, 48 +/- 3; and E+P, 49 +/- 2 nM). The increase in Vmax for transport was similar between E and E+P, indicating that exercise does not stimulate glucose transport via the beta-adrenergic receptor.

Animals↗

PubMed1988

Characterization of rat skeletal muscle sarcolemmal insulin receptors and a sarcolemmal insulin binding inhibitor.

When insulin receptors of rat skeletal muscle sarcolemmal vesicles were solubilized with Triton X-100, the specific binding of 125I-labeled insulin increased by more than 10-fold over that seen in the intact vesicles. Partial purification of the skeletal muscle insulin receptors on wheat germ agglutinin affinity columns increased the total insulin binding activity by 7-fold and reduced the Kd for insulin binding from 1.92 to 0.20 nM, suggesting that an inhibitor of insulin binding was removed by this purification step. This was confirmed when the unbound fractions of the affinity column were dialyzed and reconstituted with the insulin receptors. The inhibitory activity in the sarcolemmal extract could not be accounted for by the presence of Triton X-100. The skeletal muscle inhibitor was more potent in inhibiting insulin binding to skeletal muscle insulin receptors than to liver or adipose receptors. The inhibitor was very effective in inhibiting insulin binding to wheat germ agglutinin-purified IM-9 receptors, but had negligible effects on insulin binding to intact IM-9 cells. The properties of the alpha and beta subunits of the skeletal muscle insulin receptors appear to be the same as those of insulin receptors of other tissues: cross-linking of 125I-labeled insulin to the receptor revealed a band of 130,000 daltons, and insulin stimulated the phosphorylation of bands of 90,000 and 95,000 daltons in the receptor preparation. The skeletal muscle insulin binding inhibitor elutes from molecular sieves in a major 160,000-dalton peak and minor 75,000-dalton peak. The binding inhibitor is not inactivated by heat, by mercaptoethanol, or by trypsin, pepsin, or proteinase K. Collectively, these data suggest that the inhibitor may be a small molecule that aggregates with itself, with larger proteins, or with detergent micelles.

Adipose Tissue↗

PubMed1988

Peripheral insulin sensitivity as modified by diet and exercise training.

To determine which component of a high-fat sucrose diet (HFS) caused insulin resistance and whether exercise training or fiber could prevent it, six dietary treatments were tested in rats: low-fat complex carbohydrate (LFCC); high-fat complex carbohydrate (HFCC); low-fat sucrose (LFS); high-fat sucrose (HFS); HFS plus fiber (HFS + F); and HFS plus exercise training (HFS + EX). After 10 wk rats were subjected to an intravenous glucose-tolerance test. The HFS and HFS + F groups developed glucose intolerance, as indicated by significantly greater areas under their glucose curves compared with the LFCC group's areas. The LFS, HFS, HFS + F, and HFS + EX groups developed insulin resistance, as indicated by significantly greater areas under their insulin curves compared with the LFCC and HFCC groups' areas. Either the presence of sucrose or the absence of complex carbohydrates, not high fat, was responsible for the insulin resistance and it was not improved by adding fiber to the diet or by exercise training.

Animals↗

PubMed1988

Mechanism of insulin action on glucose transport in rat skeletal muscle.

This study was designed to examine the effect of insulin stimulation on glucose transport in rat skeletal muscle. Sarcolemmal vesicles (SL) were isolated from the gastrocnemius-plantaris and quadriceps muscles from insulin-stimulated and control groups. The insulin-stimulated group received an intravenous insulin injection (1 U/kg) 10 min before isolation. The early time course of specific D-glucose transport was linear through 2 s. Michaelis-Menten kinetics at 1.5 s indicated that the Vmax for glucose transport was increased after insulin stimulation compared with controls (4,424 +/- 668 vs. 1,366 +/- 124 pmol.mg protein -1.s-1), whereas the Km remained unchanged (19.4 +/- 0.6 vs. 21.6 +/- 3.1 mM). Scatchard plots for the D-glucose-inhibitable class of cytochalasin B binding sites indicated that insulin stimulation increased the number of binding sites in the SL vesicles (9.3 +/- 0.6 vs. 5.5 +/- 0.3 pmol/mg protein) without altering the Kd (48 +/- 3 vs. 46 +/- 3 nM). That the increase in Vmax was greater than the increase in cytochalasin B binding sites indicates that insulin stimulation caused an increase in the turnover rate of existing transport molecules as well as an increase in the total number of SL glucose transport molecules.

Animals↗

PubMed1987

Effect of diet on insulin binding and glucose transport in rat sarcolemmal vesicles.

The purpose of this study was to compare the effects of a high-fat, high-sucrose diet (HFS) and a low-fat, high-complex carbohydrate diet (LFC) on glucose tolerance, insulin binding, and glucose transport in rat skeletal muscle. During the intravenous glucose tolerance test, peak glucose values at 5 min were significantly higher in the HFS group; 0-, 20-, and 60-min values were similar. Insulin values were significantly higher in the HFS group at all time points (except 60 min), indicating whole-body insulin resistance. Skeletal muscle was responsible, in part, for this insulin resistance, because specific D-glucose transport in isolated sarcolemmal (SL) vesicles under basal conditions was similar between LFC and HFS rats (35 +/- 5 vs. 32 +/- 4 pmol/mg protein), despite the higher plasma insulin levels. Scatchard analyses of insulin binding curves to sarcolemmal vesicles revealed that the Ka of the high-affinity binding sites was significantly reduced by the HFS diet (0.63 +/- 0.09 vs. 0.35 +/- 0.05 X 10(9) M-1); no other binding changes were noted. Specific D-glucose transport in SL vesicles after maximum insulin stimulation (1 U/kg) was significantly depressed in the HFS group (87 +/- 7 vs. 58 +/- 7 pmol/mg protein), indicating that HFS feeding also caused a postbinding defect. These results indicate that the insulin resistance in skeletal muscle associated with a HFS diet is due to both a decrease in the Ka of the high-affinity insulin receptors and a postbinding defect.

Animals↗

PubMed1986

Effect of training on insulin binding to rat skeletal muscle sarcolemmal vesicles.

We examined the hypothesis that the exercise training-induced increase in skeletal muscle insulin sensitivity is mediated by adaptations in insulin binding to sarcolemmal (SL) insulin receptors. Insulin binding studies were performed on rat skeletal muscle SL isolated from control and trained rats. No significant differences were noted between groups in body weight or fat. An intravenous glucose tolerance test showed an increase in whole-body insulin sensitivity with training, and specific D-glucose transport studies on isolated SL vesicles indicated that this was due in part to adaptations in skeletal muscle. Enzyme marker analyses revealed no differences in yield, purity, or contamination of SL membranes between the two groups. Scatchard analyses indicated no significant differences in the number of insulin binding sites per milligram SL protein on the high-affinity (15.0 +/- 4.1 vs. 18.1 +/- 6.4 X 10(9)) or on the low-affinity portions (925 +/- 80 vs. 884 +/- 106 X 10(9)) of the curves. The association constants of the high-affinity (0.764 +/- 0.154 vs. 0.685 +/- 0.264 X 10(9) M-1) and of the low affinity sites (0.0096 +/- 0.0012 vs. 0.0102 +/- 0.0012 X 10(9) M-1) also were similar. These results do not support the hypothesis that the increased sensitivity to insulin after exercise training is due to changes in SL insulin receptor binding.

Animals↗

PubMed1985

Insulin binding and glucose transport in rat skeletal muscle sarcolemmal vesicles.

A new method is described for isolation of sarcolemma (SL) from skeletal muscle of rats that produces vesicles of high purity and yield. There was a mean 59-fold purification (n = 22) of the SL marker enzyme K+-p-nitrophenylphosphatase. Specific activities of marker enzymes for sarcoplasmic reticulum and mitochondria were low, indicating minimal contamination. Despite the high purity and low contamination, a relatively high protein yield was achieved (0.43 +/- 0.03 mg/g wet wt, n = 25). Electron microscopy showed that the membranes were primarily vesicles. Specific 125I-insulin binding association constants derived from the high- and low-affinity portion of the Scatchard plots were 0.764 +/- 0.154 and 0.0096 +/- 0.0012 X 10(9) M-1, whereas the apparent number of receptors were 15.0 +/- 4.1 and 925 +/- 80 X 10(9) per mg of SL protein. Equilibrium exchange glucose transport studies at 37 degrees C indicated that the SL vesicles exhibited specific D-glucose transport which was responsive to in vivo insulin stimulation. We conclude that this isolation procedure, especially in light of the high purity and yield, provides a good and practical experimental model for studying insulin binding and glucose transport in skeletal muscle.

Animals↗

PubMed1981

Effect of exhaustive exercise on myocardial performance.

Several investigators have reported ultrastructural changes in hearts of animals exercised to exhaustion. The present study was designed to determine whether functional changes occur in the intact heart at exhaustion. Adult mongrel dogs (n = 8) were chronically instrumented to measure cardiac output, coronary blood flow, aortic blood pressure, left ventricular pressure, +dP/dtmax, and -dP/dtmax. After recovery, the dogs were run to exhaustion at a constant work load, eliciting approximately 70% of maximum heart rate. The exhaustive bouts were terminated when the animals either refused or were unable to continue running, at which time their rectal temperatures approaches 42.2 degree C. The mean exhaustion time was 76.7 +/- 11.8 min. All parameters increased from rest to steady state with the exception of stroke volume (23.2 +/- 4.9 vs. 20.5 +/- 1.6 ml), which remained constant. In the transition from steady state to exhaustion, only +dP/dtmax (6,652 +/- 291 vs. 7,689 +/- 479 Torr/s) and -dP/dtmax (4,110 +/- 227 vs. 4,890 +/- 215 Torr/s) increased significantly; all other values exhibited no significant change. Similarly, when maximum cardiovascular parameters were measured before and after exhaustion, no significant changes were found. These data show that cardiac contractile function is not depressed in dogs as a result of exhaustive exercise.

Animals↗

PubMed1979

Physiological characteristics of sprint and endurance Masters runners.

Data were obtained from 13 sprint (age range 41-58) and 13 distance (age range 4-78) Masters track athletes. The mean VO2max value for the distance runners was 54.4 +/- 3 ml kg min compared with 47.2 +/- 2 ml kg min for the sprinters. The highest VO2max (71.0 ml kg min) was obtained on a 45-year-old distance runner while the lowest (27.3 ml kg min) was obtained on the oldest (78 years) distance runner. VO2max decreased by 34.5% from age 40 to 70 in the distance runner in spite of continued training that ranged from 40 to 120 miles/week. Maximum ventilation rates were 122.5 +/- 6.8 and 116.8 +/- 7.0 1 min for the sprint and distance runners, respectively. Percentage of body fat was 16.5 +/- 0.5 for the sprinters and 18.0 +/- 1.1 for the distance runners. Serum cholesterol values were 218.7 +/- 8.7 and 203.0 +/- 13.8 ml/dl, while triglyceride values were 101.5 +/- 8.2 and 84.1 +/- 9.3 mg/dl for the sprint and distance groups, respectively. These data indicate that, VO2max decreases significantly with aging despite the continuation of long distance training. Percent body fat and serum lipid levels were significantly lower in these athletes compared to those for sedentary adults, suggesting a protective effect against coronary heart disease. Only one subject had S-T segment depression during the exercise test.

Adult↗

PubMedDate not supplied

Cardiovascular response to sudden strenuous exercise.

Cardiac responses to sudden strenuous exercise were studied in seven dogs instrumented to measure cardiac output (Q), stroke volume (SV), left ventricular (LV) pressure, aortic pressure and circumflex coronary blood flow (CBF). The dogs were run on a treadmill at 12.9 km/h, 20% grade for 15s without prior warm-up. During the first 10s of the run, HR and Q increased rapidly. By 2s, myocardial O2 requirements, as estimated from either the tension time index (TTI) or calculated wall tension, had increased significantly above resting values. Simultaneously, mean CBF fell 13% and stroke CBF fell 49% below resting values at the onset of sudden exercise (2s) and was associated with 17% and 21% decreases in positive and negative LV dP/dt, a 9% decrease in (dP/dt)/P and an 11% decrease in SV. After the initial decrease at 2s, these variables then increased rapidly as the run continued except for stroke CBF which never reached resting levels. HR, Q, and LV dP/dt reached near maximal values for this level of exercise by 10s after the start of exercise. In contrast, mean CBF was still increasing when the run was terminated. These data show that sudden strenuous exercise results in a transient decrease in myocardial O2 delivery at a time when myocardial O2 requirements are rapidly increasing. This imbalance between myocardial O2 supply and demand is due to a transient decrease in coronary blood flow followed by a delay in metabolic regulation of CBF.

Animals↗