Heart rate changes caused by varying the oxygen supply to isolated hind legs of rats

Analysis of the cardiovascular changes induced in the rat by graded levels of systemic hypoxia

1. In rats anaesthetized with Saffan, we have further analysed the respiratory, cardiac and regional vascular responses induced by 3 min periods of graded hypoxia (breathing 15, 12, 8 or 6% O2 in N2). 2. Frequently, hypoxia evoked an episode, lasting 1-1.5 min, of tachycardia, renal and mesenteric vasoconstriction and skeletal muscle vasodilatation. The tachycardia and muscle vasodilatation persisted after vagotomy indicating they were not initiated by pulmonary stretch receptors secondary to hyperventilation. We propose that such episodes represented the cardiovascular components of the alerting-defence response initiated by activation of the brain stem defence areas by peripheral chemoreceptors. 3. Each of these episodes was superimposed upon gradual hyperventilation, tachycardia, fall in arterial pressure and vasodilatation in renal, mesenteric and muscle circulation the magnitudes of which at 2 min were generally graded with the level of hypoxia. In the 3rd minute, respiration and heart rate tended to wane below control levels. 4. Vagotomy had little effect on the heart rate changes and only slightly reduced the peripheral vasodilatation allowing the conclusion that the gradual tachycardia and peripheral vasodilatation was not a reflex initiated by pulmonary stretch receptors. 5. Guanethidine given after vagotomy abolished the tachycardia indicating it was sympathetically mediated; possible initiating factors are discussed. But the secondary bradycardia persisted indicating it reflected the direct effect of hypoxia on cardiac pacemaker tissue. 6. The peripheral vasodilatation persisted after guanethidine or phentolamine indicating it was mainly attributable to the local vasodilator effects of tissue hypoxia. 7. It is proposed that the components of the alerting response are an integral part of the response to systemic hypoxia. Further, that in the rat this response is superimposed upon, but may be overcome by the direct effects of hypoxia on peripheral vasculature, heart and central nervous system.

Cardiorespiratory reflex responses to static contraction of vascularly isolated hindleg muscles of the rat

One hindleg of an anasthetized rat (n = 15) was isolated from systemic blood circulation. The preparation was connected to the body only by nerve and bone. A. and V. femorales were cannulated and perfused with normoxic (PO2 = 530 mm Hg) or hypoxic (PO2 = 60 mm Hg) Tyrode solutions. Static contractions of the muscle were elicited by electrical stimulation on the sciatic nerve (2 x motor threshold, 400-800 mV, 50 s-1). A 1 s stimulus was followed by a 2 s rest period. Total test time amounted to 40 min. It was proceeded and succeeded by 20 min periods of control perfusions without stimulation. Heart rate (HR) and respiratory rate (f) were measured and cross correlated with the following outflow parameters from V. femoralis of the experimental muscle: [K+], [Na+], PO2, PCO2, pH and [lactate]. During the test period HR and f increased significantly within 20 min of the start of stimulation: HR 5.8% (p less than 0.005) and f 24.3% (p less than 0.005) for hypoxic perfusion (n = 6) and HR 3.2% (p less than 0.005) and f (p less than 0.001, ANOVA) for normoxic perfusion (n = 3). The dynamic changes of several outflow parameters were nearly simultaneous with the cardiorespiratory responses. Cross correlation analyses revealed an excellent temporal relationship between HR and PO2 or [lactate] and between f and PO2 or [lactate]. In addition PCO2 and pH correlated well with HR as well as with f. Comparison of the threshold of the cardiorespiratory response revealed an optimal relationship to pH, a good one to PCO2 and lactate concentration but no correlation to PO2.(ABSTRACT TRUNCATED AT 250 WORDS)

Respiratory and cardiac responses to exercise-simulating peripheral perfusion in endurance trained and untrained rats. II. Temporal relationships between outflow parameters and cardiac and respiratory responses

In endurance trained (TR) and untrained (UTR) rats heart rate (HR) and respiratory rate (RR) were recorded during perfusion of the circulatorily isolated hind leg of the rat with exercise simulating modified tyrode solutions (TR:n = 10, UTR:n = 10; compare part I). During the 20 min test period and the preceding and succeeding periods of control perfusions with an unmodified tyrode solution, [lactate], pH, [K+], [Na+], PO2 and PCO2 were measured in the outflow of the femoral vein. In 3 experimental series: (1) hypoxic tyrode solution enriched with lactic acid (15 mmol.l-1), (2) normoxic solution with lactic acid, (3) hypoxic solution without lactic acid, were applied. The outflow parameters were cross correlated with both HR and RR. The analysis revealed a significant temporal relationship between [lactate], pH, PO2, PCO2 and [K+] and both HR and RR. In the trained rats no temporal correlation between either of the outflow and reflex parameters could be determined. This result was not due to low [lactate], but was also found during perfusion with lactic acid. In all 3 test conditions [lactate] in untrained individuals was best correlated with both HR and RR. Although the correlation peaks of the respiratory response, but not of the HR response were definitely lower in normoxic lactic and perfusion than in the two other experimental conditions, both inter- and intraindividual correlation analyses revealed a high degree of interdependence between respiratory and cardiac responses.

Respiratory and cardiac responses to exercise-simulating peripheral perfusion in endurance trained and untrained rats. I. Reflex responses and changes in perfusion outflow

Ventilatory and circulatory drives elicited by exercise-simulating perfusion of the circulatory isolated hindleg were examined in 10 trained (TR) and untrained (UTR) rats. TR were submitted to endurance training on a motordriven treadmill (30.min-1 at a grade of 10%, 5 days a week for 30 min). Exercise was simulated by perfusion with modified tyrode solutions: I.) hypoxic, enriched with lactic acid (15 mmol.l-1), II.) normoxic, enriched with lactic acid. III.) hypoxic without lactic acid. Perfusion was performed in anaesthetized animals through cannulae in the femoral artery and vein; the hindled was connected to the rest of the body only by nerve and bone. 10 min of control perfusion (normoxic tyrode solution) was followed by a 20 min test period and another 10 min control perfusion. Apart from heart rate (HR), respiratory rate (RR) and several outflow parameters were measured ([K+], [Na+], [lactate], pH, PO2, PCO2). During control period HR was slightly higher in UTR than in TR (375.5 +/- 3.9 (SE) vs. 364.1 +/- 5.5 beats/min-1, p less than 0.6 n.s.), and RR in UTR was significantly higher than those in TR (61.5 +/- 0.4 bpm vs. 55.5 +/- 3.9 breaths.min-1, p less than 0.001). During the test periods both HR and RR in UTR increased significantly while in TR they did not (e.g. in series I mean HR and RR in UTR increased by 8.9 +/- 1.2 beats.min-1 and 1.4 +/- 0.1 breaths.min-1 respectively, whereas in TR the changes were - 2.9 +/- 1.5 beats/min-1 and -0.8 +/- 0.2 breaths.min-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Response of chemosensitive nerve fibers of group III and IV to metabolic changes in rat muscles

Spike recordings were obtained with preparations of group III and IV fibers from the nervus peroneus of the rat. During the recordings the muscle was stimulated by chemical substances simulating metabolic effects of static exercise: increase of [K+], enhancement of osmolality and increase of concentrations of lactic acid and inorganic phosphates. Two experimental setups were used: in series I application was performed by a perfusion of the circulatorily isolated hindleg, and in series II a single muscle of the hindleg (musculus extensor digitorum longus) was superfused by control or test solutions. Only those fiber preparations were further investigated which did not respond to pressure, tension or squeezing of the muscle. Only few fibers that were exposed to all of our stimuli responded to none of them; from the rest, about the half were selective or only preferential for one stimulus. The majority of the fibers adapted their response after 8 min while the applications still endured. A comparison of all fibers (in series II) proved that all the four stimuli elicited significant increases of activity. The greatest significant effects were found for lactic acid and potassium (in series I and II). Since the concentrations used in the test applications were characteristic for medium and heavy exercise these results support the hypothesis that metabolic muscle receptors participate in the peripheral control of circulatory and respiratory drives during static exercise.