1 Introduction

Diabetes mellitus is characterized by altered fluid balance, blood volume and exchangeable sodium. In metabolically stable diabetes there is an increased exchangeable sodium content (10–15%). This occurs in diabetic subjects that are normotensive or hypertensive and those with or without complications [15, 20, 48, 73]. Many of the studies on exchangeable sodium and studies in diabetics of short duration without complications suggest that sodium retention occurs early in the disease and may be relatively common [15, 20, 48, 73]. It has been suggested that the diabetics that develop nephropathy are those who are unable to compensate for this sodium retention. With the kidney disease deteriorating with time the final outcome is often the development of hypertension. These facts, taken together, indicate that the regulation of fluid balance, particularly sodium handling, early during diabetes is of great importance in the overall long-term cardiovascular complications of the diabetic state. O'Hare and colleagues have reported a diminished natriuretic response to volume expansion-induced by water immersion in diabetic patients [46–50]. This altered sodium excretion in the diabetic could not be attributed to hemodynamic or hormonal changes (release of ANF, renin, and aldosterone during water immersion) [46, 47, 50]. These investigators [46–50]were unable to identify specific mechanisms for the altered sodium excretion in these patients. To determine whether a defect in neural component of the volume reflex is responsible for some of these abnormalities in the diabetic state, we have conducted a series of experiments to examine the various components of the volume reflex (Fig. 1) in the streptozotocin (STZ)-induced diabetic rats [63]at the early phase of diabetes, 2–4 weeks after STZ injection [56, 77]. Divided broadly the various components of the volume reflex are: (1) the afferent limb …