Natriuretic response to preferential plasma volume expansion in normal unanaesthetized dogs

1. The natriuretic response of normally hydrated unanaesthetized dogs to preferential plasma volume expansion was studied, comparing the response to infusion of hyperoncotic dextran in isotonic saline with that to infusion of an identical volume of isotonic saline.

2. Significant increases in urine flow, sodium excretion, sodium filtration and potassium excretion were observed with each type of infusion.

3. The changes with each type of infusion were not significantly different however, even though plasma volume increased significantly following dextran saline infusion.

4. The natriuretic response of normal dogs to preferential plasma volume expansion appears to be commensurate with the saline load infused rather than the induced plasma volume increase.

Evidence for the renal conversion of angiotensin I in the dog

Experiments were carried out on perfused canine kidneys in situ to determine if renal conversion of angiotensin I occurs. Various concentrations of decapeptide produced an immediate increase in renal vascular resistance when injected directly into the renal artery. Since the decapeptide is biologically inactive, the increased resistance was interpreted as indicating generation of angiotensin II. The extent of renal conversion of angiotensin I to angiotensin II was estimated to be approximately 19% in the normal dog. This percent conversion was reduced to about 10% in dogs maintained on high salt and DOCA for 7 to 10 days.

Renal responses to vena caval and portal venous infusions of sodium chloride in unanaesthetized dogs

1. Recent investigation has shown that sodium excretion is greater following portal venous infusion of 5% sodium chloride (w/v) than after systemic infusion in anaesthetized dogs. It was suggested that a factor of hepatic origin which is capable of depressing renal tubular reabsorption may be involved in this response. The present experiments were undertaken to determine the renal responses of conscious, unanaesthetized dogs to infusions of 5% NaCl administered systemically and by the portal venous routes.

2. No differences were observed in urine flow or sodium excretion rates in normal dogs when responses to the two routes of NaCl infusion were compared; potassium excretion was elevated following systemic as compared to portal NaCl infusion (P < 0·05). Urine flow was significantly greater (P < 0·05) in vasopressin and mineralocorticoid treated dogs following portal infusion of 5% NaCl, but no differences in electrolyte excretion were observed between routes of administration.

3. These findings are not compatible with the concept that a natriuretic factor is released from the liver consequent to elevation of portal venous sodium concentration in the conscious dog.

Influence of developed tension on myocardial potassium balance in the dog heart

Experiments were carried out to determine the effect of changing preload on myocardial potassium balance. A blood-perfused dog heart was employed. Changes in the pressure recorded from a balloon in the left ventricle were assumed to reflect directional changes in myocardial wall tension. Developed tension was altered by changing the volume of the balloon. Net potassium balance was calculated as the product of the coronary A-V plasma potassium difference and coronary plasma flow. When the ventricle was developing little tension, it usually gained potassium. When diastolic volume and thus developed tension was increased, a net loss of myocardial potassium occurred. In some experiments the hemodynamic response of the ventricle to the increase in diastolic volume suggested that the contractility of the heart was increasing while the heart was losing potassium.

Characterization of the frequency-force relationship in the dog atrium in situ

Isometric developed tension and rate of development of tension in the in situ atrium of the anesthetized dog have been shown to increase consistently with increasing heart rate over a wide range. This study thus definitely demonstrates that both the ascending and descending staircase effects occur in the dog atrium in situ. At high cardiac rates the velocity of force development may increase without a change in the extent of force development. These changes were independent of variations in aortic pressure, intactness of betaadrenergic receptors, level of cardiac catecholamine stores, cardiac innervation, or changes in coronary flow. The character of the frequency-force relationship was found to be markedly influenced by isolation of the heart with its concomitant depression of contractility.