Effect of glucocorticoids on insulin sensitivity in newborn dogs

A 3-day treatment with 4 mg/kg Betnesol in anesthetized mongrel pups, 4-55 days of age effectively raised fasting plasma glucose concentration by 23 +/- 2.9%, which was higher than that observed in adult dogs similarly treated. Possibly this was due to the lack of effectiveness of insulin in the pups which is normally produced in response to hyperglycemia. The relative lack of effectiveness of insulin induced hypoglycemia in stimulating hepatic glucose production in the newborn pups was further demonstrated by the fact that insulin administration produced a hypoglycemia which lasted much longer than in adult dogs. On the other hand Betnesol treatment was found to be effective in restoring normoglycemia following administration of insulin in these pups presumably due to its gluconeogenic effect through the liver. In general this investigation points towards the lack of development of appropriate feedback mechanisms for glucose homeostasis in the newborn pups, and that it takes a period of about 3-4 weeks after birth to develop such mechanisms.

Effect of hyperthermia on glucose homeostasis in young dogs. An experimental study

Hyperthermia was produced in healthy anaesthetized young dogs by keeping them in a theromostatically controlled chamber, and the effects on blood glucose concentration were studies. The blood glucose levels decreased significantly at body temperatures of 40.5 degrees C and 42.5 degrees C. The decrease was greater at the latter temperature. Intravenous glucose tolerance tests were performed to study the rates of glucose utilization during hyperthermia. The calculated fractional rates of disappearance of glucose (Kt values) were found to be significantly higher in dogs having a body temperature of 42.4 degrees C. The cause of hypoglycemia produced at high body temperature seems to be due to an elevated insulin secretion which increases the over all utilization of glucose by the peripheral tissues. The study of time course of hyperglycemic response following intravenous glucose tolerance tests performed at high body temperature further support the possibility of an increase in insulin secretion in dogs subjected to hyperthermia.

Cardiac acceleration by angiotensin II in dogs

The effect of angiotensin II on heart rate of dogs were studied by intravenous and intracerebroventricular routes. Following differences were noticeable when the results obtained with intracerebroventricular were compared with those obtained after intravenous administration. The onset of tachycardia was less as compared to the intravenously administered angiotensin. The dogs required to produce tachycardia was less and the magnitude of tachycardia was also greater with intracerebroventricular administration. The initial bradycardia observed with intracerebroventricular or intravenous administration of angiotensin, could be prevented by using the mechanical buffering device. The spinal cord transection at C2 level and bilateral vagotomy abolished the tachycardia. It is speculated that angiotensin acts centrally either on the hypothalamic or medullary accelerator neurones (central sympathetic structures) and produces some degree of increased adrenegic neuron discharge which is responsible for cardiac acceleratin in dogs.

Central component of the cario-accelerator action of angiotensin II in dogs

Injection of angiotensin II into a lateral cerebral ventricle (I.C.V.) or into a peripheral vein of anaesthetized dog elicited a rise in blood pressure and transient bradycardia followed by sustained tachycardia. Spinal transection at C2 and bilateral vagotomy abolished the central cardiovascular effect of I.C.V. angiotensin. However, in spinal transected dogs the usual pressor response to intravenous angiotensin was observed. Since the transient bradycardia was absent in bilaterally vagotomized dogs or in dogs with their blood pressure stabilized by means of a mechanical buffer devise it must be reflex in origin. The tachycardia was more marked in vagotomized dogs. Prior administration of a beta adrenergic receptor blocking agent propranolol, blocked the tachycardia, but the pressor response was unaffected. The cardiovascular responses to centrally administered angiotensin were practically abolished by prior treatment of dogs with reserpine or by extirpation of both adrenal glands. Thus it may be concluded that ICV angiotensin induces a centrogenic release of catecholamines from the adrenal medulla which is responsible for the cardiovascular responses.

Stimulation of glucose production by hypoxia in newborn dogs

The aim of the experiments was to examine the role of hypoxia as a possible regulator of hepatic glucose production in newborn dogs. In pups 1–30 days of age the rate of endogenous (hepatic) glucose production increased when the animals were ventilated with a gas mixture containing 10% or 5% O2. During a single 45 min exposure to hypoxia the increase in glucose production was significantly correlated with the O2 saturation in arterial blood. A second exposure to the more severe hypoxia (5% O2) following the first (10% O2) led to a second increase in the concentration of glucose in plasma and in glucose production. In pups less than 10 days of age, the increases during the second hypoxic period were only transient. During a 60 min rest following exposure to the more severe hypoxia (5% O2) basal glucose production rates and plasma concentrations were not reestablished. The metabolic clearance rate of glucose was not changed by hypoxia. There was a significant drop in plasma free fatty acid level. The increase in endogenous glucose production to hypoxia in pups indicates that the liver in the newborn animal is capable of increasing the rate of glucose production and that the known lack of a response to hypoglycemia is due to the absence of the feedback between glucose production and extracellular glucose concentration.

Relations between blood glucose and hepatic glucose production in newborn dogs

No correlation was found between the concentration of glucose in blood plasma and the rate of hepatic glucose production in newborn pups aged 2(1/2)-112 hours. During the first few hours of life the hypoglycaemic response to insulin was small or absent. Later the response seemed exaggerated mainly owing to a slow recovery to normoglycaemia. This latter observation can be entirely accounted for by the lack of a feedback mechanism between the plasma concentration and the hepatic rate of production of glucose.

Mechanism of vasopressin-induced bradycardia in dogs

In dogs anesthetized with intravenous chloralose, vasopressin was injected either intravenously or into the cerebral ventricles, and the cardiovascular effects were studied before and after stabilizing the arterial blood pressure, bilateral vagotomy, and transection of the cervical cord at the level of C 2 . Intravenous vasopressin caused a rise in arterial blood pressure and pronounced bradycardia. The bradycardia appeared to be partly due to a peripheral depressant action on the heart, probably resulting from coronary constriction and partly from reflex vagal stimulation resulting from the rise in arterial blood pressure. A direct stimulating action of vasopressin on the central cardioinhibitory neurons played only a minor role in the production of the bradycardia. Injections of vasopressin, into either a lateral or the fourth ventricle, produced an insignificant rise in arterial blood pressure but marked bradycardia. A peripheral effect of the vasopressin after its absorption into the blood stream could be excluded as the cause of the bradycardia, since an injection of vasopressin into the cerebral ventricles produced diuresis, not antidiuresis. The bradycardia was more pronounced and occurred earlier when the vasopressin was injected into the fourth rather than into a lateral ventricle. The bradycardia produced by the injection of vasopressin into the cerebral ventricles could be almost fully accounted for by a central stimulating action on the cardioinhibitory neurons in the region of the vagal nuclei, which are closer to the fourth ventricle than to the lateral ventricles.