Introduction

Regulation of body fluid homeostasis is apparently simple since daily sodium and water intake equals daily sodium and water output. However, the mechanisms enabling the body to excrete exactly the ingested amounts of water and sodium are complex and incompletely understood, albeit thoroughly investigated. The factors regulating body fluid homeostasis may grossly be divided into neural-humoral factors and physical factors. The former group comprises among others the reinin–angiotensin–aldosterone system (RAAS), arginine vasopressin (AVP), atrial natriuretic factor (ANF), prostaglandins and renal sympathetic nerve activity. The latter group is made up by physical factors regulating cardiac function, renal blood flow and plasma protein concentration and hence osmotic pressure. The position of growth hormone (GH) in this complex scenario is not yet established, but several reports suggest that GH plays a role in body fluid regulation. More than 60 years ago anterior pituitary extracts were shown to induce fluid retention in rats (Bierring & Nielsen, 1932; Lee & Schaffer, 1934). Two decades later GH-induced sodium and fluid retention was also demonstrated in humans (Ikkos, Luft & Sjogren, 1954; Ikkos, Luft & Gemzell, 1959). Since then the sodium and water retaining effects of GH has been confirmed in several studies (Hutchings et al., 1959; Beck et al., 1960; Henneman et al., 1960; Biglieri, Watlington & Forsham, 1961; Møller et al., 1991).

The fluid volumes of the body is divided into intracellular volume (ICV) in which potassium is a predominant cation and extracellular volume (ECV) dominated by sodium and anions such as Cl−. The latter is subdivided into plasma volume (PV) and interstitial volume (Ganong, 1981).