Autonomic control of heart rate and its variability during normoxia and hypoxia in emu (Dromaius novaehollandiae) hatchlings.

Heart rate variability is a common feature of the vertebrate cardiovascular system and is a consequence of variable input from the sympathetic and parasympathetic branches of the autonomic nervous system. The aim of this study was to examine the role of the autonomic nervous system in regulating heart rate and heart rate variability in 1-d-old emu hatchlings in normoxia and during exposure to 10% O2. The role of the autonomic nervous system in controlling emu heart rate and its variability was examined by blocking the action of the cholinergic and adrenergic pathways by administration of atropine and propranolol. Heart rate of 1-d-old hatchlings exhibited a significant cholinergic tone of 60 +/- 22 beats per minute (bpm) and beta-adrenergic tone of 28 +/- 17 bpm. Cholinergic tone was unchanged during hypoxic exposure (63.5 +/- 17.7 bpm), but adrenergic tone doubled to 68 +/- 15 bpm. Initially, the majority of hatchlings exhibited high frequency oscillations with a spectral peak at 0.22 +/- 0.02 Hz, associated with respiratory sinus arrhythmia. Beta-adrenergic blockade had no effect on respiratory sinus arrhythmia or spectral power in high frequency (HF; 0.1 to 0.7 Hz), low frequency (LF; 0.01 to 0.1 Hz), or total frequency (TF) ranges. After cholinergic blockade, spectral power in HF, LF, and TF ranges and respiratory sinus arrhythmia were all abolished. Hypoxia did not initially alter spectral power in the HF, LF, or TF ranges. beta-Adrenergic blockade along with hypoxia produced a significant increase in LF oscillations. A distinct LF oscillation appeared in most birds exposed to hypoxia that was abolished by cholinergic blockade. We conclude that although both the sympathetic and parasympathetic system exert a tonic influence on heart rate, the majority of HF and TF heart rate variability is mediated by the parasympathetic system in the emu hatchling. The sympathetic system contributes to LF heart rate oscillations by suppressing the influence of the parasympathetic system on LF oscillations.

Endothermic heart rate response in broiler and White Leghorn chicks (Gallus gallus domesticus) during the first two days of post-hatch life.

The embryonic modal value of heart rate (MHR) differs between broiler and White Leghorn chickens, but the initial development of cholinergic chronotropic control of embryonic heart rate (HR) does not. Thus, we hypothesized that hatchling MHR should also differ between broiler and White Leghorn strains, while the development of a physiological regulation, such as the endothermic HR response, should not be different between hatchlings of the two strains. To test this, we measured the response of HR and cloaca temperature (Tb) to alteration of ambient temperature (Ta); i.e., 35 degrees C-25 degrees C-35 degrees C, in four groups of hatchlings on Days 0 and 1 post-hatch. Fertile eggs of both strains with similar mass were incubated simultaneously in the same incubator. Eggs of broiler chickens hatched approximately 7 h earlier than White Leghorn chicken eggs. Chick mass at hatching was identical in both strains, but diverged during 2 days after hatching. Tb measured at the initial Ta of 35 degrees C was identical in both strains. MHR at the same Ta was approximately 30 bpm lower in broiler chicks than in White Leghorn chicks, but the difference was reversed to that observed in the embryos. The endothermic HR response was advanced by approximately 1 day in broiler chicks compared with White Leghorn chicks. As a result, eggs of similar mass in both strains produced chicks with similar mass and Tb at hatching, but during 2 days of post-hatch life their masses diverged and regulation of the endothermic HR response developed earlier in broiler than in White Leghorn hatchlings. This physiological heterochrony between strains is most likely due to genetic selection for fast growth in broiler chickens.

Development of endothermic metabolic response in embryos and hatchlings of the emu (Dromaius novaehollandiae).

During hatching, there is a maturation of the mechanisms controlling the respiratory physiology involved in endotherm in precocial avian species. Here we examined the timing of the development of an endothermic response of oxygen uptake (MO2) to an alteration of ambient temperature (T(a)) in a model precocial species, the preterm and hatching emu (Dromaius novaehollandiae). Late stage pre-pipped and pipped embryos and hatchlings were measured for responses of MO2 and shell or skin temperature (T(s)) to altered T(a) (DeltaT(a)). MO2 remained unchanged in pre-pipped and internally pipped (IP) embryos at the end of 1.5h exposure to DeltaT(a) of +/-10 degrees C. Externally pipped (EP) embryos responded to a cooling and a warming exposure with marked increase and decrease in MO2, as hatchlings responded to DeltaT(a) with an endothermic change in MO2. The demonstration of the endothermic inverse metabolic response first appearing in EP embryos suggests that pre-EP embryos may also possess the ability to produce the endothermic inverse metabolic response, but they are restricted by the eggshell gas conductance. Late pre-pipped and IP embryos were measured again for responses of [Formula: see text] to DeltaT(a) in air and then in a 40% O(2) environment. The metabolic response of pre-pipped embryos at 90% of incubation was partially altered by switching from air to hyperoxia. IP embryos responded to DeltaT(a) in 40% O(2) with apparent inverse changes in MO2. The late stage emu embryo possesses the ability to produce an endothermic metabolic response at an earlier stage of development than in chickens, but this response is limited by the eggshell gas conductance.

Maturation of the homeothermic response of heart rate to altered ambient temperature in developing chick hatchlings (Gallus gallus domesticus).

On the basis of evidence showing that instantaneous heart rate (IHR) of chick hatchlings responds to exposure to altered ambient temperature (Ta; Tazawa H, Moriya K, Tamura A, and Akiyama R. Comp Biochem Physiol A 131A: 797-803, 2002), we elucidate here the developmental timeline for the homeothermic response of HR in newly hatched chicks (days 0-7) maintained at room temperature ( approximately 24-27 degrees C). Hatchlings were exposed to Ta of 25, 35, and 25 degrees C for 1-h periods, respectively, and IHR was measured together with skin temperature (Ts) during this warming and cooling bout. Early 0-day-old (0 day) chicks responded to warming and cooling exposures with various changes in HR baseline. In newly hatched chicks (0-7 h old), HR baseline was elevated during warming (Delta126 beats/min, n = 13) and declined during cooling (-Delta94 beats/min). With progress of development on day 0, the elevation of HR baseline during warming decreased and advanced 0-day chicks tended to decrease HR baseline during warming rather than increase HR. The more developed 1- to 7-day-old chicks exhibited the expected homeothermic decrease in HR during warming. The diurnal variations of HR responses during warming and cooling on the first day of post-egg life indicate that pronounced development of thermoregulatory competence occurs during the day of hatching (day 0). The response of IHR fluctuations to altered Ta was observed in the form of low- and high-frequency oscillations. High-frequency oscillations corresponding to respiratory sinus arrhythmia developed as the hatchlings aged. There was a significant increase in the number of chicks exhibiting both low- and high-frequency oscillations that depended on age and the development of thermoregulatory competence of hatchlings.

Cardiac rhythms of late pre-pipped and pipped chick embryos exposed to altered oxygen environments.

During the final stages of embryonic development in chickens, diffusive gas exchange through the chorioallantoic membrane (CAM) is progressively replaced by pulmonary respiration that begins with internal pipping (IP) of the CAM. Late chick embryos going through the transition from CAM respiration to pulmonary respiration were exposed to hyperoxic (100% O(2)) and hypoxic (10% O(2)/N(2)) environments for 2-h and the responses of baseline heart rate (HR), and HR fluctuation patterns were investigated. 16- and 18-day-old (referred to as 18-d) embryos and 20-d externally pipped (EP) embryos were examined as pre-pipped embryos and pipped embryos, respectively. 19-d embryos were divided into two groups: embryos that had not yet internally pipped (Pre-IP embryos) and embryos that had internally pipped (IP embryos). IP was identified by detecting the breathing signal with a condenser microphone attached hermetically on the eggshell (i.e. acoustorespirogram) on day 19 of incubation. In the hyperoxic environment, HR baseline of pre-pipped embryos remained unchanged and that of pipped embryos was depressed. In the hypoxic environment, HR baseline of 16-d pre-pipped embryos was depressed and that of pipped (IP and EP) embryos elevated. These different responses in pipped embryos might be partially attributed to increased cholinergic input from the vagus nerve in hyperoxia and increased adrenergic response in hypoxia. While hyperoxia did not induce marked modification of instantaneous heart rate (IHR) fluctuation patterns, hypoxia tended to augment transient decelerations of IHR in late pre-pipped embryos and markedly depressed HR fluctuations in pipped embryos.

Heart rate responses to cooling in emu hatchlings.

Among fluctuations of instantaneous heart rate (IHR) in newly hatched chicks, heart rate (HR) oscillation with a mean frequency of 0.7 Hz has been designated as Type II HR variability characterized by low frequency (LF) oscillation [Comp. Biochem. Physiol. Part A 124 (1999) 461]. In response to exposure to lowered ambient temperature (Ta), chick hatchlings raised their HR baseline accompanied with the production or augmentation of Type II HR oscillation, indicating that LF oscillation is a phenomenon relating to thermoregulation [J. Therm. Biol. 26 (2001) 281]. In emu hatchlings that are precocial like chickens, Type II HR oscillation also occurred, but less frequently in comparison with chick hatchlings [Comp. Biochem. Physiol. Part A 131 (2002) 787]. This present experiment was conducted to elucidate how IHR of emu hatchlings responds to changes in Ta. Six hatchlings were measured for IHR and skin temperature (Ts) during a 3-h period when they were exposed to controlled Ta (ca. 35 degrees C), lowered Ta (ca. 15-30 degrees C) and again the controlled Ta for individual 1-h periods. In response to all the cooling and re-warming procedures, HR baseline changed depending upon the intensity of the Ta differences; i.e. large differences of Ta produced large changes in HR. HR fluctuations tended to augment during cooling with a few exceptions, but LF oscillation was not produced. Thus, LF oscillation, which was scarce even at the controlled Ta, could not be used as a thermoregulatory indicator in emus.

Physiological control of warming and cooling during simulated shuttling and basking in lizards.

Differences in warming and cooling rates in basking lizards have long been thought to be brought about by adjustments in heart rate and blood flow. We examined the physiological control of warming and cooling in Iguana iguana, Sceloporus undulatus, and three species of Cordylus by measuring time constants, heart rate, and superficial capillary blood flow. Previously, techniques have not been available to measure time constants in shuttling animals. Using a combination of rapid measurements of temperature and blood flow and numerically intensive parameter-fitting methods, we measured dominant and subdominant time constants in lizards subjected to periods of both simulated basking and simulated shuttling. Cutaneous blood flow and heart rate were measured using laser Doppler flowmeters. Of the three, only the larger I. iguana measurably altered rates of warming and cooling during basking. During shuttling, none of the species effectively controlled warming and cooling. During both basking and shuttling, blood flow and heart rate tended to change in predicted directions. Superficial blood flow correlated with surface temperature while heart rate correlated more closely with core temperature. Changes in superficial blood flow and heart rate varied relatively independently in I. iguana. The techniques used here provide a better understanding of the ability of these species to control thermoregulation.

Thermal time constant estimation in warming and cooling ectotherms.

(1) Measurement of physiological control of warming and cooling in reptiles requires calculating the thermal time constant (tau) of the animal. (2) Previously reported methods of estimating tau are sensitive to multiple problems including measurement error in operative environmental temperature and equilibrium body temperature, drift of environmental temperatures, requirements for extremely simple thermal environments, and ill conditioning of the estimation techniques themselves. (3) We propose a physiologically based heat transfer model which is less sensitive to common experimental errors, more numerically robust, and can provide physiologically meaningful estimates of time constants. (4) The method presented here allows time constants to be measured for animals subjected to the traditional step change experiment as well as to shorter periods of warming and cooling such as during shuttling.

Central mu opioids mediate differential control of urine flow rate and urinary sodium excretion in conscious rats.

Central administration of the selective mu opioid agonist, dermorphin, produces a concurrent diuretic and antinatriuretic response in conscious rats. To determine whether central mu opioids differentially affect the renal excretion of water and sodium, we examined changes in renal function produced by intracerebroventricular (i.c.v.) administration of dermorphin during continuous intravenous (i.v.) infusion of a synthetic ADH analogue in conscious Sprague-Dawley rats. During ADH infusion the typical diuresis produced by i.c.v. dermorphin was abolished although the antinatriuresis remained intact. Alone, i.v. ADH produced a decrease in urine flow rate without significantly altering urinary sodium excretion. In other studies, the effects of i.c.v. dermorphin were examined on the renal responses produced by i.v. infusion of a V2-ADH receptor antagonist. In these studies the magnitude of the V2 antagonist-induced diuresis was not altered by i.c.v. dermorphin but the increase in urinary sodium excretion produced by this antagonist was converted to an antinatriuresis. Central dermorphin did not alter heart rate or mean arterial pressure in either study. These findings suggest that the effects of central dermorphin on renal sodium and water handling are mediated by separate mechanisms; the effects on water involving changes in circulating ADH levels and the effects on sodium independent of the action of this hormone.