Ventilatory response to hypoxia in unanesthetized newborn kittens

We studied the ventilatory response to hypoxia in 11 unanesthetized newborn kittens (n = 54) between 2 and 36 days of age by use of a flow-through system. During quiet sleep, with a decrease in inspired O2 fraction from 21 to 10%, minute ventilation increased from 0.828 +/- 0.029 to 1.166 +/- 0.047 l.min-1.kg-1 (P less than 0.001) and then decreased to 0.929 +/- 0.043 by 10 min of hypoxia. The late decrease in ventilation during hypoxia was related to a decrease in tidal volume (P less than 0.001). Respiratory frequency increased from 47 +/- 1 to 56 +/- 2 breaths/min, and integrated diaphragmatic activity increased from 14.9 +/- 0.9 to 20.2 +/- 1.4 arbitrary units; both remained elevated during hypoxia (P less than 0.001). Younger kittens (less than 10 days) had a greater decrease in ventilation than older kittens. These results suggest that the late decrease in ventilation during hypoxia in the newborn kitten is not central but is due to a peripheral mechanism located in the lungs or respiratory pump and affecting tidal volume primarily. We speculate that either pulmonary bronchoconstriction or mechanical uncoupling of diaphragm and chest wall may be involved.     

Expiratory effects of vagal stimulation in newborn kittens

Expiratory effects of electrical stimulation of vagal afferents were studied in 12 kittens during the first week of life. Animals anesthetized with ketamine (30 mg/kg, im) and acepromazine (1.1 mg/kg, im), tracheostomized, and paralyzed were artificially ventilated after bilateral vagotomy. Rectified and "integrated" activity of the C5 root of phrenic nerve, systemic blood pressure, and the stimulus train were recorded. The optimal stimulus parameters for expiratory prolongation were chosen. The results varied between animals. We found three types of response: A, expiratory prolongation when stimulus was applied within the initial 80% of control expiratory time (TEc); beyond this delay, a decreased response or no effect was observed in four kittens; B, graded expiratory prolongation was recorded to the end of this phase in three kittens; and C, expiratory prolongation when stimulus delay was less than 40% of TEc and expiratory shortening when the stimulus given with greater delays was observed in one kitten. Nonsignificant effects were observed in the remaining four animals. Types A and B of response suggest activation of the slowly adapting pulmonary stretch receptors. However, amplitude of stimulus and frequency of pulses were higher than those used in adult animals. Type C response indicates that fibers from both rapidly and slowly adapting stretch receptors could be activated. Our results imply that if the expiratory insensitive phase is present in kittens, it can be affected by experimental conditions. This is in contradiction to characteristics of expiratory response to vagal stretch receptor input in adult cats.     

Density distribution and cation composition of red blood cells in newborn puppies

The density distribution and cation composition of red blood cells from newborn puppies have been studied. The density distribution of red cells from a newborn puppy in a bovine serum albumin density gradient resembles a normal distribution with a peak density at a region less than that found for adult dog red cells. In two weeks the whole distribution shifts toward a more dense region, and a second cell peak appears so that the distribution becomes bimodal. This second cell peak is smaller than the original peak, and it appears at a region of lower density. In nine weeks the distribution becomes a normal one again, but the peak density corresponds to the peak density of the second cell peak which first appeared at two weeks. Evidence has been obtained to show that fetal red cells are located in the more dense cell peak and neonatal cells are in the less dense second peak. These results were obtained by labeling fetal cells with Cr⁵¹ and neonatal cells with Fe⁵⁹. The analysis of the cation content of these cells shows that fetal cells contain more K and Na and have a higher K/Na ratio than adult red cells. Furthermore, neonatal cells contain considerably less cation and hemoglobin than do fetal cells. From a study of the cation and hemoglobin content of red cells appearing in various density fractions it is concluded that fetal cells lose K and Na during the first two weeks after birth. Thus, the change in the density disribution of the erythrocytes is thought to be due to two factors: (1) An increase in the density of fetal cells due to the loss of K and Na and, hence, water during the first two weeks after birth, and (2) the entry of less dense neonatal cells into the circulation.     

Phrenic activity and intercostal muscle emg during inspiratory loading in newborn kittens

The effects of airway occlusions at functional residual capacity (FRC) on both "integrated" phrenic activity (Phr) and intercostal muscle electromyogram (intEMG) were studied in intact and vagotomized spontaneously breathing kittens during the 1st wk of life. Animals were anesthetized im with a mixture of ketamine (30 mg/kg) and acepromazine (1.1 mg/kg). In the intact kittens, inspiratory loading led to a significant increase in peak amplitudes of both Phr and intEMG and prolongation of inspiratory (TI) and expiratory (TE) times. Mean values of rate of rise of Phr and intEMG measured at 200 ms (intEMG200) from the onset of inspiration were unaffected. The results indicated that in newborns the vagal component of the load compensation is of great importance. Following vagotomy, airway occlusion produced a significant increase in mean values of TI and intEMG only. These small but significant changes suggest that most of the load compensation reflex is dependent on prolongation of TI. Increased intEMG200 during loading in the vagotomized kittens, observed during several trials, implies that the intercostal fusion-alpha interaction may operate in newborns.     

Sodium and potassium content and membrane transport properties in red blood cells from newborn puppies

The intracellular sodium and potassium concentrations and membrane transport properties for these ions were investigated in red blood cells from newborn puppies and adult dogs. At birth the intracellular concentrations of sodium and potassium are much higher than those found in adult dog red cells. During the first few weeks of life the intracellular concentrations of these ions gradually decrease until the adult level is reached. Changes in the membrane transport properties develop concurrently. The rate of active potassium influx, as measured by ouabain-sensitivity, and the pump to leak ratio are greater in red cells from newborn puppies than in those from adult animals. No ouabain-sensitive sodium efflux could be demonstrated in red cells from older puppies or adult dogs. When either puppy or adult dog red cells are depleted of ATP (by incubation at 37°C with no substrate), potassium permeability increases, and the permeability of the membrane to sodium decreases. The addition of adenosine reverses the effect of depletion.     

Endotoxin fever in para-chlorophenylalanine (PCPA) treated newborn guinea pigs and kittens.

Guinea pigs aged 0–3 days responded to 0.2 μg icv. E. coli endotoxin with phasic changes in body temperature; two rises separated by a marked fall. This fall was diminished in PCPA treated animals. In 5–10 day-old kittens the otherwise monophasic endotoxin fever was modified by PCPA treatment; the rise was attenuated in the period corresponding to the transient temperature fall of guinea pigs.     

Prolonged suspended animation in puppies

Three groups of a total of 26 puppies were subjected to surface-induced hypothermia with or without limited left-heart bypass to seek recovery after 2.5-hr total circulatory arrest. In spite of various protective measures, surface-induced hypothermia which was carried out until the effective circulation ceased, failed to protect the animal from 2.5-hr circulatory arrest. With a combination of limited leftheart bypass, the central nervous tissue tolerated the procedure better than expected. Metabolic derangements, although severe immediately after the procedure, were reversible. Respiratory distress was a serious problem, which was considerably alleviated in the 3rd group of 10 puppies by perfusion of the lung with a solution approximating the intracellular electrolyte composition. Six puppies of this group survived the procedure, 5 without any persistent disorders. These results indicate the possibility of 2.5-hr hypothermie circulatory arrest.     

Renin disappearance rate in puppies

The disappearance rate of endogenous renin from the circulating blood after bilateral nephrectomy was studied in 4 puppies aged 16-21 days. Mean half-times of the fast and slow component of the disappearance curves of renin were 9.15 min +/- 0.87 (SD) and 84.0 min +/- 16.8 (SD), respectively. These values are not different from the values reported for adult dogs in the literature. The metabolic degradation of renin is not different between young and adult dogs and does not contribute to the elevated plasma renin activity in the puppy.