Experimental physiology
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Experimental physiology · May 2001
Comparative StudyCardiovascular response to graded lower body negative pressure in young and elderly man.
Lower body negative pressure (LBNP) reduces central venous pressure (CVP) and cardiac output. The elderly are reported to have a limited capacity to increase cardiac output by increasing heart rate (HR), are especially dependent on end diastolic volume to maintain stroke volume and therefore should be especially vulnerable to LBNP. The present study compared the effects of LBNP in the young and old. ⋯ Both groups produced similar significant increases in vascular resistance, HR, plasma vasopressin (AVP) and noradrenaline. Mean arterial blood pressure (MBP) and plasma adrenaline did not change significantly. Therefore healthy old men respond to LBNP in a similar manner to the young, although MBP and SD are regulated around different baselines in the two groups.
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Experimental physiology · Jan 2001
Raised extracellular potassium attenuates the sympathetic modulation of sino-atrial node pacemaking in the isolated guinea-pig atria.
Intense exercise or myocardial ischaemia can significantly increase both the concentration of extracellular potassium ([K(+)](o)) and cardiac sympathetic nerve activity. Since changes in [K(+)](o) modulate membrane currents involved in sino-atrial node pacemaking, in particular the voltage-sensitive hyperpolarization-activated current (I(f)), we investigated whether raised [K(+)](o) (from 4 mM to 8 or 12 mM) could directly affect the heart rate response to cardiac sympathetic nerve stimulation (SNS). In the isolated guinea-pig atrial-right stellate ganglion preparation, raised [K(+)](o) significantly decreased the maximum diastolic potential, amplitude and maximum rate of rise of the upstroke of sino-atrial node pacemaker action potentials in 8 and 12 mM [K(+)](o) (P < 0.05). ⋯ In conclusion, extreme physiological levels of [K(+)](o) attenuate the positive chronotropic response to cardiac sympathetic activation due to decreased activation of the I(f) current. This is consistent with raised [K(+)](o) protecting the myocardium from potentially adverse effects of excessive noradrenaline. Experimental Physiology (2001) 86.1, 19-25.
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Experimental physiology · Nov 2000
Changes in chemoreflex characteristics following acute carbonic anhydrase inhibition in humans at rest.
The effect of carbonic anhydrase (CA) inhibition with acetazolamide (ACZ, 10 mg kg(-1) I. V.) on the peripheral and central chemosensitivity and breathing pattern was investigated in four women and three men aged 25 +/- 3 years using a modified version of Read's rebreathing technique. Subjects were exposed to dynamic increases in CO2 in hypoxic and hyperoxic backgrounds during control conditions and following acute CA inhibition. ⋯ Following ACZ administration, the peripheral chemosensitivity was blunted (control vs. ACZ treatment: 3.66 +/- 0.92 vs. 1.33 +/- 0.46 l x min(-1) (mmHg CO2)(-1), P < 0.05). In conclusion, acute CA inhibition enhanced the central chemosensitivity to CO2 but diminished the peripheral chemosensitivity.
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Experimental physiology · May 2000
Clinical TrialThermoregulation in winter swimmers and physiological significance of human catecholamine thermogenesis.
Thermoregulation in control subjects and cold-adapted winter swimmers was examined during 1 h of cold water immersion (13 C). It was found that the thermoregulatory functions of winter swimmers differ from those of non-cold-adapted subjects. As evident from the relationship between rectal temperature and the magnitude of cold thermogenesis, in controls a significant part of cold thermogenesis during the early phase of cooling was induced by changes in peripheral temperature input, while in the late phase of cooling it was the central temperature input which was mainly engaged in induction of cold thermogenesis. ⋯ Additionally, the capacity of the total cold thermogenesis due to potentiation of non-shivering heat production was also increased. Heat produced due to thermogenic action of adrenaline may represent more than a quarter of the total cold thermogenesis. In conclusion, the data suggest that winter swimmers exhibit metabolic, hypothermic and insulative types of cold adaptation.
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Experimental physiology · Jul 1999
Effect of hypocalcaemia on glucose metabolism in hyperketonaemic piglets.
In nine 2- to 3-month-old hyperketonaemic piglets the kinetics of glucose and D-beta-hydroxybutyrate (D-BHB) metabolism were studied during hypo- and normocalcaemia in paired experiments. Hyperketonaemia (1.3 and 2.5 mmol D-BHB (l plasma)-1) was generated by a stepwise increase of DL-BHB infusion. Hypocalcaemia spontaneously developed in five piglets due to an inherited calcitriol deficiency and was induced in four control piglets by a continuous infusion of Na2-EDTA. ⋯ In the presence of hyperketonaemia, hypocalcaemia increased the rate of replacement of D-BHB by 6-40%. The replacement rate represents the sum of endogenous production and the rate of DL-BHB infusion. This observation shows that the endogenous production of D-BHB was higher during hypocalcaemia than during normocalcaemia.