The APS Journal Legacy Content is the corpus of 100 years of historical scientific research from the American Physiological Society research journals. This package goes back to the first issue of each of the APS journals including the American Journal of Physiology, first published in 1898. The full text scanned images of the printed pages are easily searchable. Downloads quickly in PDF format.

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To determine the effects of different anesthesias on the performance of the arterial baro-reflex, the open-loop characteristic of the carotid sinus reflex was analyzed in 24 rabbits under anesthesia with pentobarbital (30 mg/kg), urethan (800 mg/kg), alpha-chloralose (80 mg/kg), or a mixture of alpha-chloralose (40 mg/kg) and urethan (0.4 g/kg). For each rabbit and anesthesia, mean systemic arterial pressure and heart rate were measured as carotid sinus pressure was changed in 10-mmHg steps between 40 and 150 mmHg. This set of measurements was repeated four times at 1-h intervals. A logistic function curve was fitted to the carotid sinus pressure-arterial pressure relationship. The parameters of this curve were then analyzed to delineate the specific effects of the anesthesias on the relationship. The main finding was that the response range and the slope parameters under alpha-chloralose anesthesia were significantly smaller than those obtained under the other anesthesias. Propylene glycol, used as the solvent for chloralose, did not affect the reflex control of arterial pressure or heart rate. The reflex under chloralose-urethan anesthesia showed characteristics similar to those under urethan anesthesia. We conclude that although alpha-chloralose has traditionally been used in the dog to obtain strong reflex responses, it weakens the reflex control of arterial pressure in the rabbit.

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SUMMARY. We studied the interaction between the carotid sinus and aortic arch baroreflexes' control of arterial pressure in 13 rabbits anesthetized with urethane. The carotid sinus baroreceptor region was isolated to control carotid sinus pressure. The aortic nerve was transected cervically and stimulated with 4-V electrical pulses at 5 Hz and 0.5-msec duration, which caused a 20-30 mm Hg decrease in arterial pressure. Several protocols were used to analyze the interaction. In the first, we compared the combined effect of the stimulation of a unilateral aortic nerve and a simultaneous increase of bilateral carotid sinus pressures from 40 to 70-80 mm Hg to the sum of these effects measured separately. The combined effect was always larger than the sum of the separate effects. In the second protocol, we varied carotid sinus pressures between 40 and 140 mm Hg and, at each carotid sinus pressure, measured the decrease in arterial pressure caused by unilateral aortic nerve stimulation. The fall in arterial pressure was always greater in the middle range of carotid sinus pressure (70-90 mm Hg) than in the lower or higher range. In the third protocol, we increased the intensity of aortic nerve stimulation from 2 to 4 V while raising carotid sinus pressure from 90 to 110 mm Hg. At this increased tonic input level, the combined effect was always smaller than the sum of the separate effects. Thus, we observed a facilitatory summation when the input variables changed from subthreshold to physiological level, but an inhibitory summation when the input stimulus intensity increased from a physiological to a supraphysiological level. We conclude that interaction between the two reflexes depends in part upon the initial level of the input intensity to the receptors. THE combined effect of two reflex inputs given simultaneously can be equal to, greater than, or less than the sum of their effects given separately. The first case is defined as additive summation (i.e., no interaction), the second facilitatory (synergistic) summation, and the third inhibitory (occlusive) summation. In the recent literature concerning sino-aortic baroreflex summation, all of the three types of summation have been reported. Angell-James and Daly (1970) compared the combined and separate effects of changing pressures in the isolated carotid sinus and aortic arch on reflex control of systemic arterial pressure in dogs on heart-lung bypass. They found that the reflex effect of combined stimulation was smaller than the sum of the effects of separate stimulations. Thus, they concluded that there is inhibitory summation between the two reflexes. In contrast, Donald and Edis (1971) found the sum of the two reflex effects on perfused hindlimb pressure to be approximately equal to their combined effect. Therefore, these authors concluded that the summation is additive. In a similarly pump-perfused hindlimb vascular bed of the rabbit, Guo et al. (1982) found that the two reflex controls of vascular resistance were almost completely redundant, i.e., a strongly ...