Biomedical subjects

J Shenberger

Publications and source records attributed to J Shenberger.

4 recordsLinked to original sources

PubMed1996

Forearm training attenuates sympathetic responses to prolonged rhythmic forearm exercise.

We previously demonstrated that nonfatiguing rhythmic forearm exercise at 25% maximal voluntary contraction (12 2-s contractions/min) evokes sympathoexcitation without significant engagement of metabolite-sensitive muscle afferents (B.A. Batman, J.C. Hardy, U.A. Leuenberger, M.B. Smith, Q.X. Yang and L.I. Sinoway. J. Appl. Physiol. 76: 1077-1081, 1994). This is in contrast to the sympathetic nervous system responses observed during fatiguing static forearm exercise where metabolite-sensitive afferents are the key determinants of sympathetic activation. In this report we examined whether forearm exercise training would attenuate sympathetic nervous system responses to rhythmic forearm exercise. We measured heart rate, mean arterial blood pressure (MAP), muscle sympathetic nerve activity (microneurography), plasma norepinephrine (NE), and NE spillover and clearance (tritiated NE kinetics) during nonfatiguing rhythmic forearm exercise before and after a 4-wk unilateral forearm training paradigm. Training had no effect on forearm mass, maximal voluntary contraction, or heart rate but did attenuate the increase in MAP (increase in MAP: from 15.2 +/- 1.8 before training to 11.4 +/- 1.4 mmHg after training; P < 0.017), muscle sympathetic nerve activity (increase in bursts: from 10.8 +/- 1.4 before training to 6.2 +/- 1.1 bursts/min after training; P < 0.030), and the NE spillover (increases in arterial spillover: from 1.3 +/- 0.2 before training to 0.6 +/- 0.2 nmol.min-1.m-2 after training, P < 0.014; increase in venous spillover: from 2.0 +/- 0.6 before training to 1.0 +/- 0.5 nmol.min-1.m-2 after training, P < 0.037) seen in response to exercise performed by the trained forearm. Thus forearm training reduces sympathetic responses during a nonfatiguing rhythmic handgrip paradigm that does not engage muscle metaboreceptors. We speculate that this effect is due to a conditioning-induced reduction in mechanically sensitive muscle afferent discharge.

Adult↗

PubMed1989

Muscle acidosis during static exercise is associated with calf vasoconstriction.

In this study we measured (n = 6) the phosphocreatine-to-inorganic phosphate ratio (PCr/Pi), Pi, and pH with 31P-nuclear magnetic resonance (31P-NMR) in the human forearm during static work at 30% of maximal voluntary contraction (MVC) for 2 min followed immediately by 3 min of circulatory arrest (forearm arterial occlusion). Static exercise, with its central volitional and skeletal muscle metabolic and mechanical afferent components, caused a rise in heart rate (HR, 32%), blood pressure (BP, 29%), and calf vascular resistance (calf R, 30%). During forearm occlusion after static exercise, HR returned to base line, the increase in BP was attenuated by 30%, and calf R remained elevated and unchanged. The percent change in calf R was correlated with forearm cellular pH (R = 0.56, P less than 0.001) but only weakly associated with PCr/Pi (R = 0.33, P less than 0.042). 30% MVC for 1 min followed by arterial occlusion (3 min) reduced PCr/Pi by 65% and pH by 0.16 U (P less than 0.05). Calf R was unchanged. Circulatory arrest alone (20 min) caused no change in either pH or calf R but large changes in PCr/Pi (50% reduction). We conclude that 1) there is an association between forearm cellular acidosis and calf vasconstriction during static forearm exercise and 2) large changes in PCr/Pi without concomitant changes in pH are not associated with changes in calf R.

Acidosis↗

PubMed1988

Sympathetic tone affects human limb vascular resistance during a maximal metabolic stimulus.

To evaluate the relationship between heightened sympathetic tone and maximal metabolic vasodilation, peak forearm blood flow (ml.min-1.100 ml-1) was measured plethysmographically in 18 volunteers after the release of 10 min of arterial occlusion (the peak reactive hyperemic blood flow response, RHBF) both before and after a stimulus to induce heightened sympathetic tone. The stimulus was the application of ice to the forehead for 90 s just before and during RHBF measurements. Mean arterial cuff blood pressure (MAP; mmHg) was calculated, and corresponding resistance (R; mmHg.ml-1.min.100 ml) was derived from blood pressure divided by RHBF. During ice application, blood pressure rose (pre 92 vs. post 115 ml/mmHg; P less than 0.05), peak RHBF was unchanged (pre 38.8 vs. post 36.4 ml.min-1.100 ml-1; not significant), but R during the maximal metabolic stimulus rose (pre 2.5 vs. post 3.2 mmHg.ml-1.min.100 ml; P less than 0.05). To examine the effects of heightened sympathetic tone on conduit vessels, simultaneous measurements of maximal metabolic blood flow (RHBF) and brachial artery Doppler velocity (V, cm/s) were conducted (n = 5) with and without ice applied to the forehead. Velocity rose by 70% as flow remained constant. Thus brachial artery area (area = flow/velocity) and diameter decreased substantially (20% decrease in diameter). The increase in R noted with ice was due to an alpha-mediated response, since the increase in R was blocked by oral prazosin (n = 6) and was unaffected by maneuvers to alter myogenic tone (n = 5). We conclude that maximal metabolic vasodilation can be counteracted to some extent by the effects of heightened sympathetic tone. Moreover, some of the interaction between these two opposing influences takes place at the arterial level.

Adult↗

PubMed1987

A 30-day forearm work protocol increases maximal forearm blood flow.

To evaluate the local circulatory changes that accompany chronic localized work, we studied the effects of a 4-wk handgrip work protocol on maximal forearm work-related blood flow (ml X min-1 X 100 ml-1) in the nondominant forearms of six normal subjects. The reactive hyperemic blood flow response (RHBF) was also evaluated pre- and posttraining in both forearms of each subject to determine whether maximal vasodilatory capacity would be enhanced. In addition, maximal O2 consumption (VO2max) was measured. We found that chronic handgrip work led to an increase in work-related blood flow (before, 22.4; after, 32.1; P less than 0.05); a drop in work-related minimal resistance (R) (before, 6.4; after, 4.1; P less than 0.05). RHBF rose in the chronically exercised extremity by 30% (before, 33.5; after, 43.7; P less than 0.05) as minimal R fell (before, 3.2; after, 2.2; P less than 0.05). RHBF and R in the unstimulated dominant forearm remained unchanged (blood flow: before, 33.5; after, 31.0; NS; R before, 3.2; after, 3.2; NS). VO2max (ml X kg-1 X min-1) did not change (before, 35.7; after, 34.0). These findings show that localized skeletal muscle forearm work is associated with a localized increase in vasodilation (RHBF). Thus the vascular system appears to be an independent integral partner in the training process.

Adult↗