Lung growth in response to altered metabolic demand in hamsters: influence of thyroid function and cold exposure

Metabolism, temperature, and ventilation

In mammals and birds, all oxygen used (VO2) must pass through the lungs; hence, some degree of coupling between VO2 and pulmonary ventilation (VE) is highly predictable. Nevertheless, VE is also involved with CO2 elimination, a task that is often in conflict with the convection of O2. In hot or cold conditions, the relationship between VE and VO2 includes the participation of the respiratory apparatus to the control of body temperature and water balance. Some compromise among these tasks is achieved through changes in breathing pattern, uncoupling changes in alveolar ventilation from VE. This article examines primarily the relationship between VE and VO2 under thermal stimuli. In the process, it considers how the relationship is influenced by hypoxia, hypercapnia or changes in metabolic level. The shuffling of tasks in emergency situations illustrates that the constraints on VE-VO2 for the protection of blood gases have ample room for flexibility. However, when other priorities do not interfere with the primary goal of gas exchange, VE follows metabolic rate quite closely. The fact that arterial CO2 remains stable when metabolism is changed by the most diverse circumstances (moderate exercise, cold, cold and exercise combined, variations in body size, caloric intake, age, time of the day, hormones, drugs, etc.) makes it unlikely that VE and metabolism are controlled in parallel by the condition responsible for the metabolic change. Rather, some observations support the view that the gaseous component of metabolic rate, probably CO2, may provide the link between the metabolic level and VE.

Effects of thyroxine and naloxone administration on metabolism and ventilation in hamsters

This study examined the interaction between hyperthyroidism and opioid receptor function on control of ventilation and metabolism in male Harlan hamsters 4 and 8 weeks after implanting thyroxine (T(4)) or placebo pellets. Metabolism, but not body temperature, increased in T(4)-treated hamsters relative to placebo-treated animals. After 4 weeks, body weights were greater in the T(4)-treated hamsters, but comparable to controls after 8 weeks. At that time, body length was greater in T(4)-treated hamsters than in controls. Thyroxine did not affect ventilation in air or in response to CO(2). Naloxone, an opioid receptor antagonist, decreased metabolism in T(4)-treated, but not in placebo-treated hamsters without affecting ventilation in air in either group. In the placebo group naloxone augmented the ventilatory response to hypercapnia by increasing frequency. These results negate our hypotheses that: (1) hyperthyroid hamsters exhibit greater ventilation in air and in response to hypercapnia than controls; and (2) that naloxone augments these effects.

Structural abnormalities underlying alveolar hypoventilation and fluid imbalance in the dystrophic hamster lung

Bio 14.6 dystrophic hamsters exhibit alveolar hypoventilation and increased lung hydration. This study evaluated whether age- and genotype-related morphometric differences in lungs exist and correlate with the development of lung pathophysiology. Morphometry was used to characterize lungs of young (Y) and mature (M) control (C) and dystrophic (D) hamsters. With age, both C and D had increased barrier surface area [S(a-b,p)] and morphometric diffusing capacity index [mdci], and decreased harmonic thickness. In C but not D, mean capillary diameter [d(c)] and parenchymal volume density [V(v)(p,L)] increased with age, whereas barrier arithmetic thickness decreased. Chord length increased with age, whereas the ratio of parenchymal surface area to airspace volume [S/V] and the intersection density of the air-blood interface [I(v)(a-b,s)] decreased in D but not C. At both ages, lung volume relative to body mass was greater in D than C. With that exception, no genotype differences were found in young hamsters. Mature D displayed lower V(v)(p,L), S/V, d(c), I(v)(a-b,s), S(a-b,p), and mdci than mature C. Independent of age, chord length was greater but arithmetic thickness, airspace surface density, frequency of type II cells, and lamellar body area and volume density were lower in D than C. We conclude: 1) lung volume relative to body growth was greater in dystrophics than controls; 2) parenchymal remodeling was delayed or abnormal in dystrophics; 3) lower diffusing capacity in mature dystrophics may effect alveolar hypoventilation; 4) lower tissue volume, surface area, and the type II cell abnormalities in dystrophics could reduce sodium and water transport leading to greater lung hydration.

Sexual dimorphism in the architecture of the lung's gas-exchange region

The lung's only vital function is to provide sufficient gas-exchange surface area (Sa) to meet the organism's needs for oxygen uptake (VO2) and carbon dioxide elimination. A direct linear relation between Sa and VO2 and an inverse linear relation between the size of the lung's gas-exchange units and the species mass-specific VO2 are strongly conserved across species. Within species, Sa increases in response to prolonged (weeks) elevation of VO2. We now report sex-dependent deviations from these relationships that seem to anticipate the need for increased gas-exchange capacity engendered in females by the metabolic demands of pregnancy and lactation. We found that although VO2 almost doubled in rats during pregnancy and lactation, Sa was the same in age-matched virgin, pregnant, and lactating females. However, at the onset of sexual maturity, virgin female rats and mice had higher mass-specific Sa than males of the same species although mass-specific VO2 was identical, within species, in both sexes. In addition, even though mass-specific VO2 was identical in males and females, alveoli were 30% and 50% smaller in female rats and mice, respectively, than males of the same species. We suggest the greater mass-specific Sa and smaller alveoli in females in spite of identical mass-specific VO2 as males were selected for evolutionarily; they help females meet the metabolic demands of reproduction without adding to the energy demands of these periods a requirement to form additional lung.