Alan W. Pinder

Temperature and the Balance between Aerial and Aquatic Respiration in Larvae of Rana berlandieri and Rana catesbeiana

Tadpoles of the frogs Rana berlandieri and Rana catesbeiana increased total oxygen uptake through lungs, gills, and skin as temperature increased. However, the partitioning of oxygen uptake among lungs, gills, and skin was similar at all experimental temperatures (15, 25, and 33 C), with the skin being the major site of O₂ uptake. This constancy of partitioning in tadpoles differs from the increased predominance of pulmonary respiration with rising temperatures reported for many other amphibious vertebrates. We suggest that neither the costs nor the physical constraints of aquatic O₂ exchange limit its predominance at high temperatures in tadpoles and perhaps in other forms.

Larval Cardiorespiratory Ontogeny and Allometry in Xenopus laevis

Very little is known about the early development of cardiorespiratory regulatory mechanisms in newly hatched amphibian larvae. We tested whether early cardiovascular responses to hypoxia reflect local-flow regulation in tissues and whether regulation of ventilation would improve during larval development. Cardiac output was calculated from heart rate and stroke volume, and buccal pumping rate was measured at 19° -21° C for Xenopus laevis larvae between Nieuwkoop and Faber stages 44 (just after hatching) and 57 (4-1,102 mg) denied access to air at a range of ambient aquatic Po₂ from normoxia (150-155 mmHg) to severe hypoxia (27-45 mmHg). Cardiac output decreased in severe hypoxia in stage 44-49.5 larvae, but not in stage 51-54 larvae, because heart rate decreased significantly in the early larvae, probably a direct effect of O₂ limitation on cardiac metabolism. Stroke volume did not change significantly in hypoxia in either early- or late-stage larvae. Thus there was no evidence of a tissue-mediated increase in cardiac output in hypoxia. Buccal pumping increased by about 50% over normoxic rates in moderate hypoxia in all larvae but sharply decreased in severe hypoxia, decreasing more in younger larvae than older. Younger larvae show significantly more variability in buccal pumping than older larvae, which suggests that regulatory mechanisms are not yet fully developed in early larvae. Cardiac output scales to body mass with a allometric coefficient of 1.15 ± 0.15 (95% confidence limits), significantly higher than literature values for O₂ uptake (0.83), implying that cardiovascular gas transport may be less important (compared to direct diffusion) in very small early-stage larvae than in larger, late-stage larvae.

Air Exposure and Physiological Compensation in a Tropical Intertidal Chiton, Chiton stokesii (Mollusca: Polyplacophora)

The duration of natural intertidal air exposure was determined for a population of Chiton stokesii from the rocky intertidal zone of Naos Island, Panama. On-site measurement indicated that chitons seek protected locations during air exposure and are little affected by increased air temperatures. Construction of an artificial tidal system allowed hemolymph sampling and assessment of physiological compensation of chitons during air exposure in the laboratory. During air exposure chitons showed significant hemoconcentration associated with reduction in heart rate and intracardiac hemolymph pressure. Air exposure was also associated with the buildup of hemolymph Pco2 and the development of significant but largely compensated hemolymph acidosis. No consequent decrease in postbranchial hemolymph oxygenation occurred; indeed, the partial pressure of O2 in postbranchial hemolymph (Pao2) had increased slightly but significantly at 17 h air exposure. This increased oxygenation was not associated with significant change in of O2 uptake (Ṁo2), but this was significantly enhanced following air exposure, which suggests that an O2 debt may have been incurred. This chiton can tolerate air exposure of a duration greater than three times that normally experienced, can maintain O2 consumption in airpossibly by O2 uptake across the air-exposed gills, and appears adapted to exploit rather than simply survive air exposure in its natural intertidal habitat.

Gravitational gradients and blood flow patterns in specialized arboreal (Ahaetulla nasuta) and terrestrial (Crotalus adamanteus) snakes.

Abstract Blood pressure and blood flow patterns were recorded from the carotid artery and aortae of a thick-bodied terrestrial snake (Crotalus adamanteus) and a thin-bodied arboreal species (Ahaetulla nasuta) anesthetized with ketamine hydrochloride. Hemodynamic stress induced by rotation resulted in pronounced changes in the blood flow patterns and pressure in C. adamanteus: rotation of A. nasuta produced changes of a similar type, but of a much lower magnitude. The markedly different responses of these two species, the baroreceptor reflexes of which were disrupted, suggest that morphological factors – such as differential gross cardiac displacement, or variation in the interaortic foramen – in addition to physiological factors, are important in determining a snakes ability to withstand hemodynamic stress.

Heart-Rate and Hemolymph Pressure Responses to Hemolymph Volume Changes in the Land Crab Cardisoma-Guanhumi - Evidence for Baroreflex Regulation

Hemolymph volume in restrained land crabs (Cardisoma guanhumi) was altered in 5% increments by rapid infusion of filtered seawater into, or rapid withdrawal of hemolymph from, an infrabranchial sinus. Associated changes in intracardiac hemolymph pressure and in heart rate were measured. Elevation of hemolymph volume resulted in a brief (<20 s) rise in hemolymph pressure followed by a return toward control levels. Heart rate changed in a reciprocal fashion to hemolymph pressure, briefly decreasing during and immediately following infusion and then rising back toward control levels. The net effect after infusion or withdrawal was a readjustment in both hemolymph pressure and heart rate to stable new levels. A significant inverse correlation between heart rate and hemolymph pressure was evident, with heart rate decreasing approximately 2 beats · min⁻¹ · mmHg⁻¹ rise in hemolymph pressure. Collectively, these data indicate that changes in hemolymph pressure result in reflex adjustments in heart rate in a manner analogous to baroreceptor reflexes in vertebrates.

Gas exchange in isolated perfused frog skin as a function of perfusion rate.

Patches of skin were removed from bullfrogs and perfused with a red cell suspension through the cutaneous artery to define the gas exchange characteristics of frog skin without complications caused by in vivo regulatory mechanisms. Oxygen uptake was primarily perfusion limited at low perfusion rates but primarily diffusion limited at perfusion rates that were similar to cutaneous blood flows previously reported in vivo. Diffusing capacity (DO2) increased only slightly as perfusion rate increased. Because the DO2 in isolated skin, in which DO2 should be maximal, was not significantly higher than estimates of DO2 in vivo, there seems to be little opportunity for increasing cutaneous DO2 or oxygen uptake in vivo.

An Isolated Perfused Frog Skin Preparation for the Study of Gas Exchange

The vascular arrangement of frog skin offers two advantages for studying gas exchange within the microcirculation, both due to the relatively simple two-dimensional geometry of the skin: 1) all vessels are visible, thus amenable to techniques being developed for analysing blood flow from video images, and 2) oxygen uptake, gas partial pressure gradients, perfusion rate, perfusate capacitance etc. can be easily manipulated. Other tissues have much more complex three dimensional vasculatures in which it is difficult to observe an entire capillary bed supplied by a particular vessel. Tissues that are thin enough to observe red cell flow through the entire capillary bed (mesentery, thin muscles) have too low an oxygen consumption to simultaneously measure oxygen delivery. Thus, it is difficult to test models of oxygen delivery being developed to predict the effects of microcirculatory red cell flow patterns on gas exchange. Although the isolated perfused frog skin preparation was developed specifically for studying cutaneous oxygen uptake, it has promise for studying more general features of gas exchange in capillary beds, such as the effects of blood flow heterogeneity and red cell spacing on oxygen uptake and delivery.