The effect of β-adrenoceptor blockade on body temperature and plasma renin activity in heat-exposed man

Adaptations and mechanisms of human heat acclimation: Applications for competitive athletes and sports

Exercise heat acclimation induces physiological adaptations that improve thermoregulation, attenuate physiological strain, reduce the risk of serious heat illness, and improve aerobic performance in warm-hot environments and potentially in temperate environments. The adaptations include improved sweating, improved skin blood flow, lowered body temperatures, reduced cardiovascular strain, improved fluid balance, altered metabolism, and enhanced cellular protection. The magnitudes of adaptations are determined by the intensity, duration, frequency, and number of heat exposures, as well as the environmental conditions (i.e., dry or humid heat). Evidence is emerging that controlled hyperthermia regimens where a target core temperature is maintained, enable more rapid and complete adaptations relative to the traditional constant work rate exercise heat acclimation regimens. Furthermore, inducing heat acclimation outdoors in a natural field setting may provide more specific adaptations based on direct exposure to the exact environmental and exercise conditions to be encountered during competition. This review initially examines the physiological adaptations associated with heat acclimation induction regimens, and subsequently emphasizes their application to competitive athletes and sports.

Effects of short-term thermal stress on plasma catecholamine concentrations and plasma renin activity in pregnant and nonpregnant women

OBJECTIVE

The purpose of this study was to find out whether the effects of thermal stress on plasma catecholamines and plasma renin activity are altered by pregnancy.

STUDY DESIGN

Nonpregnant (n = 15) and pregnant (13 to 14 weeks, n = 23; 36 to 37 weeks, n = 23) healthy women were examined. Samples were taken before the exposure, in the heat chamber (70 degrees C), and during the recovery. The BMDP statistical software was used.

RESULTS

The concentration of epinephrine in plasma increased significantly only in the nonpregnant group. The epinephrine concentrations in the pregnancy groups increased most in the subjects who felt discomfort during or after the exposure. Plasma norepinephrine concentrations increased without significant differences between the groups. Plasma renin activity increased in each group, but the increases were significantly lower in the pregnancy groups as compared with the nonpregnant group.

CONCLUSIONS

Pregnancy does not alter the thermally induced increase in norepinephrine release. The adrenaline response is mostly dependent on the mental discomfort. The plasma renin activity response to thermal stress is blunted during pregnancy.

Adrenalectomy abolishes antagonism of alpha-adrenoceptor-mediated hypotension by a beta-blocker in conscious rats

1. The effects of a single bolus injection of propranolol, atenolol or ICI 118,551, non-selective beta-, selective beta 1- and selective beta 2-adrenoceptor antagonists, respectively, on mean arterial pressure (MAP) and plasma catecholamine concentrations were examined in seven groups of conscious and unrestrained adrenalectomized rats receiving a continuous infusion of the alpha-adrenoceptor antagonist phentolamine. In all rats adrenaline was undetectable in the plasma four days after adrenalectomy. 2. In the first three groups, phentolamine significantly decreased MAP and increased the plasma concentrations of noradrenaline. The injection of propranolol, atenolol or ICI 118,551 in Groups I, II and III, respectively, caused a small increase in MAP and a small, but not significant, decrease in plasma noradrenaline concentrations. 3. Groups IV, V and VI were given a continuous infusion of adrenaline for 1 h prior to the infusion of phentolamine, followed by a bolus injection of propranolol, atenolol or ICI 118,551, respectively. Group VII was treated similarly to IV, but was also given daily cortisone replacement after adrenalectomy. Adrenaline slightly, but not significantly, decreased MAP while phentolamine significantly decreased MAP and increased plasma noradrenaline concentrations in all groups. A subsequent injection of a beta-blocker caused a significant increase in MAP in each group and a slight decrease in the plasma level of noradrenaline which reached statistical significance in group VII. The pressor effect of propranolol was significantly greater in the cortisone-treated rats (Group VII) than in Group IV. 5. The results suggest that adrenaline and adrenocortical steroids are both involved in the antagonism of alpha-adrenoceptor-induced hypotension by a beta-blocker.

Effect of selective and nonselective beta-adrenoceptor blockade on thermoregulation during prolonged exercise in heat

The effect of selective and nonselective beta-adrenoceptor blockade on the thermoregulatory responses of 11 physically active, healthy, young adult men was studied during 2-hour block-stepping in heat. The trial consisted of 3 periods of 6 days each during which propranolol (160 mg/day), atenolol (100 mg) or matching placebo was administered in a randomized, double-blind crossover fashion. Propranolol and atenolol induced similar, significant (p less than 0.001) increases in subjective ratings of perceived exertion. The mechanism of this increased fatigue was not evident from the documented alterations in serum electrolyte, blood glucose and blood lactate levels or ventilatory parameters. Propranolol did, however, induce a postexercise delayed serum-potassium reversion. Although rectal and mean skin temperature responses were essentially unaltered by beta-adrenoceptor blockade during block-stepping, an increased total sweat loss was observed with propranolol (p less than 0.01 versus placebo) and to a lesser degree with atenolol (p = not significant versus placebo). This indicates that persons receiving beta-adrenoceptor blockers have an increased need to adhere to a strict fluid-replacement regimen during exercise. This potentially adverse response was minimal with atenolol in contrast to propranolol, and this in turn suggests the use of beta1-selective adrenoceptor blockers during prolonged exercise when adequate fluid replacement is not possible.

Cardiovascular and respiratory function values in hyperthermia: the effect of beta-receptor blockade with bunitrolol

During hyperthermia in water, the rectal temperature of 12 healthy young test subjects rose by on average 1.16 degrees C. Under treatment with placebo, the pulse rate increased by 40.6 beats/min, systolic pressure by 14.6 mm Hg, oscillatory respiratory resistance fell by 0.29 hPa/1/s and time-averaged flow ventilation increased by 3.62 1/min. Under treatment with a beta-receptor blocker (10 mg of bunitrolol) there was no change in the tachycardiac response, there was slightly less of a rise of blood pressure, slightly less of a fall of oscillatory respiratory resistance and time-averaged flow ventilation remained low (2p less than 0.05). The response of the cardiovascular parameters and oscillatory respiratory resistance is discussed in connection with redistributions of blood volume during hyperthermia. The less pronounced hyperventilation is considered to be due to the effect of beta-receptor blockade in warding off external stress-induced factors.

Some aspects of the pharmacology of beta-adrenorecptor blockers

The pharmacodynamic properties of a beta-blocker are mainly determined by its affinity to beta1 and beta2-receptors respectively and by its intrinsic activity. It is suggested that there is no absolute organ separation of the two receptor sub-types. Instead both beta1 and beta2-receptors are involved in the mediation of the same effect. The frequency distribution ratio of beta1/beta2-receptors varied markedly among various effector responses. A non-selective and a beta1-selective blocker may have different haemodynamic effects when the levels of circulating adrenaline are high, because of their markedly different potency in inhibiting the beta2-mediated vasodilator effect of adrenaline. Data are presented which suggest the existence of a presynaptic beta1-receptor mediating a positive feedback mechanism on neuronal release of noradrenaline.