Atrial receptors, vasopressin and blood volume in the dog.

Recent work has clarified the relationship between stimulation of left atrial receptors and plasma vasopressin concentration (pAVP) and has allowed a rational explanation of a number of previously anomalous findings. There is now good evidence that mitral obstruction causes a decrease in pAVP and that the decreases in pAVP can occur within a normal range of pAVP in anaesthetized and unanaesthetized animals. A stimulus which is localised to the left atrial receptors also causes a decrease in pAVP and it is likely that this is due to stimulation of the complex unencapsulated endings in the atrium, with myelinated afferent fibres. Evidence is lacking that changes in the stimulus to ventricular receptors or to cardio-pulmonary receptors with C-fibre afferents influences pAVP. The diuretic response to left atrial distension is two-fold, an increase in free water clearance and a natriuresis. The increase in free water clearance is due to the decrease in pAVP; the cause of the natriuresis is unknown. The changes in pAVP occur rapidly in response to atrial distension (within 5 min). The stimulus provided to atrial receptors by atrial distension and the decrease in pAVP is maintained for at least 90 min. pAVP is also modulated in response to small changes in blood volume (+/- 10%). The changes in pAVP that occur over this range of blood volume are likely to be in the range of 1-10 pg/ml and to have their effects on renal water excretion rather than on vascular resistance. The much larger changes in pAVP which occur with greater degrees of blood loss, and which can affect vascular resistance are likely to be produced by changes in the stimulus to other receptors, but a low input from atrial receptors may be permissive for these stimuli to be effective. More work is needed to clarify the relationship between inputs from different receptor types.