Influence of plasma osmolality on baroreflex control of sympathetic activity

The purpose of this study was to determine if plasma osmolality alters baroreflex control of sympathetic activity when controlling for a change in intravascular volume; we hypothesized that baroreflex control of sympathetic activity would be greater during a hyperosmotic stimulus compared with an isoosmotic stimulus when intravascular volume expansion was matched. Seven healthy subjects (25 +/- 2 yr) completed two intravenous infusions: a hypertonic saline infusion (HSI; 3% NaCl) and, on a separate occasion, an isotonic saline infusion (ISO; 0.9% NaCl), both at a rate of 0.15 ml x kg(-1) x min(-1). To isolate the effect of osmolality, comparisons between HSI and ISO conditions were retrospectively matched based on hematocrit; therefore, baroreflex control of sympathetic outflow was determined at 20 min of a HSI and 40 min of an ISO. Muscle sympathetic outflow (MSNA) was directly measured using the technique of peroneal microneurography; osmolality and blood pressure (Finometer) were assessed. The baroreflex control of sympathetic outflow was estimated by calculating the slope of the relationship between MSNA and diastolic blood pressure during controlled breathing. Plasma osmolality was greater during the HSI compared with the ISO (HSI: 292 +/- 0.9 mosmol/kg and ISO: 289 +/- 0.8 mosmol/kg, P < 0.05). Hematocrits were matched (HSI: 39.1 +/- 1% and ISO: 39.1 +/- 1%, P > 0.40); thus, we were successful in isolating osmolality. The baroreflex control of sympathetic outflow was greater during the HSI compared with the ISO (HSI: -8.3 +/- 1.2 arbitrary units x beat(-1) x mmHg(-1) vs. ISO: -4.0 +/- 0.8 arbitrary units x beat(-1) x mmHg(-1), P = 0.01). In conclusion, when controlling for intravascular volume, increased plasma osmolality enhances baroreflex control of sympathetic activity in humans.     

Influence of isometric exercise on blood flow and sweating in glabrous and nonglabrous human skin.

The distribution of the reflex effects of isometric exercise on cutaneous vasomotor and sudomotor function is not clear. We examined the effects of isometric exercise by different muscle masses on skin blood flow (SkBF) and sweat rate (SR) in nonglabrous skin and in glabrous skin. The latter contains arteriovenous anastomoses (AVAs), which cause large fluctuations in SkBF. SkBF was measured by laser-Doppler flowmetry (LDF) and reported as cutaneous vascular conductance (CVC; LDF/mean arterial pressure). SR was measured by capacitance hygrometry. LDF and SR were measured at the sole, palm, forearm, and ventral leg during separate bouts of isometric handgrip (IHG) and isometric leg extension (ILE). CVC and its standard deviation decreased significantly during IHG and ILE in the palm and sole (P < 0.05) but not in the forearm or leg (P > 0.05). Only palmar SR increased significantly during IHG and ILE (P < 0.05). We conclude that the major reflex influences of isometric exercise on the skin include AVAs and palmar sweat glands and that this is true for both arm and leg exercise.     

Relationship between skin blood flow and sweating rate, and age related regional differences

To examine the mechanisms and regional differences in the age-related decrement of skin blood flow, 11 young (age 20-25 years) and 10 older (age 64-76 years) men were exposed to a mild heat stress by immersing their feet and lower legs in water at 42 degrees C for 60 min, while they were sitting in near thermoneutral conditions [25 degrees C and 45% relative humidity (rh)]. During the equilibrium period (25 degrees C and 45% rh) before the heat test, no group differences were observed in rectal (Tre) and mean skin (Tsk) temperatures or mean arterial pressure (MAP). During passive heating, Tsk was significantly lower in the older men 20 min after commencing exposure (P<0.001), although there were similar increases in Tre in both groups. Exposure time and age did not affect MAP. The local sweating rate (m(sw)) and the percentage change in skin blood flow by laser Doppler flowmetry (%LDF) relative to baseline values on the chest, back, forearm and thigh were significantly lower in the older men (P<0.001), especially on the thigh. After starting the heat exposure, three temporal phases were observed in the relationship between %LDF and m(sw) at most sites in each subject. In phase A, %LDF increased but with no increase in m(sw). In phase B, m(sw) increased but with no secondary increase in %LDF. Finally, in phase C, there were proportional increases in %LDF and m(sw). The increase in %LDF in phase A was significantly lower on the forearm and thigh (P<0.05) for the older men, but not on the chest and back. In phase C, the slopes of the regression lines between %LDF and m(sw) were lower for the older men on the back (P<0.03), forearm (P = 0.08) and thigh (P<0.03), but not on the chest. These results would suggest that the age-related decrement in skin blood flow in response to passive heating may be due in part to a smaller release of vasoconstrictor tone and to less active vasodilatation once sweating begins. Regional differences exist in the impaired vasoconstriction and active vasodilatation systems.     

Role of skin blood flow and sweating rate in exercise thermoregulation after bed rest.

Two potential mechanisms, reduced skin blood flow (SBF) and sweating rate (SR), may be responsible for elevated intestinal temperature (T(in)) during exercise after bed rest and spaceflight. Seven men underwent 13 days of 6 degrees head-down bed rest. Pre- and post-bed rest, subjects completed supine submaximal cycle ergometry (20 min at 40% and 20 min at 65% of pre-bed rest supine peak exercise capacity) in a thermoneutral room. After bed rest, T(in) was elevated at rest (+0.31 +/- 0.12 degrees C) and at the end of exercise (+0.33 +/- 0.07 degrees C). Percent increase in SBF during exercise was less after bed rest (211 +/- 53 vs. 96 +/- 31%; P < or = 0.05), SBF/T(in) threshold was greater (37.09 +/- 0.16 vs. 37.33 +/- 0.13 degrees C; P < or = 0.05), and slope of SBF/T(in) tended to be reduced (536 +/- 184 vs. 201 +/- 46%/ degrees C; P = 0.08). SR/T(in) threshold was delayed (37.06 +/- 0.11 vs. 37.34 +/- 0.06 degrees C; P < or = 0.05), but the slope of SR/T(in) (3.45 +/- 1.22 vs. 2.58 +/- 0.71 mg x min-1 x cm-2 x degrees C-1) and total sweat loss (0.42 +/- 0.06 vs. 0.44 +/- 0.08 kg) were not changed. The higher resting and exercise T(in) and delayed onset of SBF and SR suggest a centrally mediated elevation in the thermoregulatory set point during bed rest exposure.     

Influence of menstrual cycle on shivering, skin blood flow, and sweating responses measured at night

In 10 women, external cold and heat exposures were performed both in the middle of luteal phase (L) and in the early follicular phase (F) of the menstrual cycle. Serum progesterone concentrations in L and F averaged 46.0 and 0.9 nmol X l-1, respectively. The experiments took place between 3:00 and 4:30 A.M., when the L-F core temperature difference is maximal. At neutral ambient temperature, esophageal (Tes), tympanic (Tty), rectal (Tre), and mean skin (Tsk) temperatures averaged 0.59 degrees C higher in L than in F. The thresholds for shivering, chest sweating, and cutaneous vasodilation (heat clearance technique) at the thumb and forearm were increased in L by an average of 0.47 degrees C, related to mean body temperature [Tb(es) = 0.87Tes + 0.13 Tsk] and to Tes, Tty, Tre, or Tsk. The above-threshold chest sweat rate and cutaneous heat clearances at the thumb and forearm were also enhanced in L, when related to Tb(es) or time. The metabolic rate, arm blood flow, and heart rate at thermoneutral conditions were increased in L by 5.0%, 1.1 ml X 100 ml-1 X min-1, and 4.6 beats X min-1, respectively. The concomitant increase in threshold temperatures for all autonomic thermoregulatory responses in L supports the concept of a resetting of the set point underlying the basal body temperature elevation in L. The effects of the increased threshold temperatures are counteracted by enhanced heat loss responses.     

Changes in blood plasma osmolality and states of mania

Manic patients were studied systematically over a period of months. Fluctuations in their blood plasma osmolality were observed and these correlated significantly and inversely with undulations in their mania, rating scores. Successful treatment of patients with lithium carbonate was correlated with increasing levels of their plasma osmolality and with a significant remission of mania symptomatology. Animal studies have shown that adaptations to altered blood plasma osmolalities are accompanied by profound metabolic changes in tissues of the central nervous system. A similar relationship is likely to exist in man. We suggest, therefore, that an altered plasma osmolality is linked to changes in manic symptomatology, through osmotically regulated metabolic changes in the central nervous system.     

Heat acclimation increases skin vasodilation and sweating but not cardiac baroreflex responses in heat-stressed humans.

In the present study, to test the hypothesis that exercise-heat acclimation increases orthostatic tolerance via the improvement of cardiac baroreflex control in heated humans, we examined cardiac baroreflex and thermoregulatory responses, including cutaneous vasomotor and sudomotor responses, during whole body heating before and after a 6-day exercise-heat acclimation program [4 bouts of 20-min exercise at 50% peak rate of oxygen uptake separated by 10-min rest in the heat (36 degrees C; 50% relative humidity)]. Ten healthy young volunteers participated in the study. On the test days before and after the heat acclimation program, subjects underwent whole body heat stress produced by a hot water-perfused suit during supine rest for 45 min and 75 degrees head-up tilt (HUT) for 6 min. The sensitivity of the arterial baroreflex control of heart rate (HR) was calculated from the spontaneous changes in beat-to-beat arterial pressure and HR. The HUT induced a presyncopal sign in seven subjects in the preacclimation test and in six subjects in the postacclimation test, and the tilting time did not differ significantly between the pre- (241 +/- 33 s) and postacclimation (283 +/- 24 s) tests. Heat acclimation did not change the slope in the HR-esophageal temperature (Tes) relation and the cardiac baroreflex sensitivity during heating. Heat acclimation decreased (P < 0.05) the Tes thresholds for cutaneous vasodilation in the forearm and dorsal hand and for sweating in the forearm and chest. These findings suggest that short-term heat acclimation does not alter the spontaneous baroreflex control of HR during heat stress, although it induces adaptive change of the heat dissipation response in nonglabrous skin.     

Blood flow and catecholamine concentration in bovine and caprine skin during thermal sweating

1. Blood flow and the concentrations of noradrenaline, adrenaline and dopamine were determined in the skins of cattle and goats, before, at the onset of and 3 hr after commencement of sweating induced by heat exposure (40 degrees C). 2. The onset of sweating in both cattle and goats was associated with a rise in cutaneous blood flow, which was thus independent of sweat pattern. Cutaneous blood flow was also higher at 40 degrees C than at 15 degrees C. 3. The predominant catecholamine in the skin of both species was dopamine, which in the goat increased in concentration in the warm environment. 4. There was no clear evidence of a change in the amount of any of the cutaneous catecholamines during exposure to 40 degrees C, although there was a consistent tendency for the concentrations of adrenaline in the calf and noradrenaline in the goat, to fall during the onset of sweating.     

Local sweating and cutaneous blood flow during exercise in hypobaric environments

The effect of acute hypobaric hypoxia on local sweating and cutaneous blood flow was studied in four men and four women (follicular phase of menstrual cycle), who exercised at 60% of their altitude-specific peak aerobic power for 35 min at barometric pressures (PB) of 770 Torr (sea level), 552 Torr (2,596 m), and 428 Torr (4,575 m) at an ambient temperature of 30 degrees C. We measured esophageal temperature (Tes), mean skin temperature (Tsk, 8 sites), and local sweating (ms) from dew-point sensors attached to the skin at the chest, arm, and thigh. Skin blood flow (SkBF) of the forearm was measured once each minute by venous occlusion plethysmography. There were no gender differences in the sensitivity (slope) or the threshold of either ms/Tes or SkBF/Tes at any altitude. No change in the Tes for sweating onset occurred with altitude. The mean slopes of the ms/Tes relationships for the three regional sites decreased with increasing altitude, although these differences were not significant between the two lower PBS. The slope of SkBF/Tes was reduced in five of the eight subjects at 428 Torr. Enhanced body cooling as a response to the higher evaporative capacity of the environment is suggested as a component of these peripheral changes occurring in hypobaric hypoxia.     

Effect of plasma flow on the regeneration of skin wounds and body reactivity

The local application of plasma flow to postsurgery wounds in rabbits has revealed accelerated wound healing, reduced perifocal inflammation, the appearance of great numbers of phagocytes and islands of epithelial cells in the wound and certain stimulation of hemopoietic body function. The suppression of free-radical oxidation was accompanied by the activation of the metabolic processes. All this justifies an attempt to apply "plasma methods" in the clinical practice for the treatment of long-healing wounds.     

Effect of plasma exchange on blood viscosity and cerebral blood flow.

The effects of plasma exchange using a low viscosity plasma substitute on blood viscosity and cerebral blood flow were investigated in eight subjects with normal cerebral vasculature. Plasma exchange resulted in significant reductions in plasma viscosity, whole blood viscosity, globulin and fibrinogen concentration without affecting packed cell volume. The reduction in whole blood viscosity was more pronounced at low shear rates suggesting an additional effect on red cell aggregation. Despite the fall in viscosity there was no significant change in cerebral blood flow. The results support the metabolic theory of autoregulation. Although changes in blood viscosity appear not to alter the level of cerebral blood flow under these circumstances, plasma exchange could still be of benefit in the management of acute cerebrovascular disease.