Question

1. Compare the effect of ANP to aldosterone with respect to what nephron cells are stimulated to do (indicate what transport proteins are involved, where they go, and what their functions are)?
2. How would the volume and total concentration of urine change in the presence of BOTH aldosterone and ADH? Explain your answer.
3. What does aldosterone stimulate the nephron cells to do (indicate what transport proteins are involved, where they go, and what their functions are)?

Answer 1

Answer;-Aldosterone Increases Sodium Reabsorption and Potassium Secretion. Aldosterone, secreted by the zona glomerulosa cells of the adrenal cortex, is an important regulator of sodium reabsorption and secretion of potassium and hydrogen ions by the renal tubules. A major renal tubular site of aldosterone action is on the principal cells of the cortical collecting tubule. The mechanism by which aldosterone increases sodium reabsorption and potassium secretion is by stimulating the sodiumpotassium ATPase pump on the basolateral side of the cortical collecting tubule membrane. Aldosterone also increases the sodium permeability of the luminal side of the membrane. The most important stimuli for aldosterone are (1) increased extracellular potassium concentration and (2) increased angiotensin II levels, which typically occur in conditions associated with sodium and volume depletion or low blood pressure. The increased secretion of aldosterone associated with these conditions causes renal sodium and water retention, helping to increase extracellular fluid volume and restore blood pressure toward normal. In the absence of aldosterone, as occurs with adrenal destruction or malfunction (Addison’s disease), there is marked loss of sodium from the body and accumulation of potassium. Conversely, excess aldosterone secretion, as occurs in patients with adrenal tumors (Conn’s syndrome), is associated with sodium retention and decreased plasma potassium concentration due, in part, to excessive potassium secretion by the kidneys. Although day-to-day regulation of sodium balance can be maintained as long as minimal levels of aldosterone are present, the inability to appropriately adjust aldosterone secretion greatly impairs the regulation of renal potassium excretion and potassium concentration of the body fluids. Thus, aldosterone is even more important as a regulator of potassium concentration than it is for sodium concentration.

Function of ANP:-ANP in the circulation act on the kidneys to increase Na excretion, this action is just opposite to the aldosterone function.they appear to produce this effect by dilating atterent arterioles and relaxing, mesangial-cells. Both of these actions increase glomerular filtration . In addition, they act on the renal tubules to inhibit-Na reabsorpton. Other actions include an increase in capillary permeability, leading to extravasation of fluid and a decline in blood pressure. In addition, they relax vascular smooth muscle in arterioles and venules.

Role of ADH IN URINE CONCENTRATION:-ADH Increases Water Reabsorption. The most important renal action of ADH is to increase the water permeability of the distal tubule, collecting tubule, and collecting duct epithelia. This effect helps the body to conserve water in circumstances such as dehydration. In the absence of ADH, the permeability of the distal tubules and collecting ducts to water is low, causing the kidneys to excrete large amounts of dilute urine, a condition called diabetes insipidus. Thus, the actions of ADH play a key role in controlling the degree of dilution or concentration of the urine, ADH binds to specific V2 receptors in the late distal tubules, collecting tubules, and collecting ducts, increasing the formation of cyclic adenosine monophosphate and activating protein kinases. This action, in turn, stimulates the movement of an intracellular protein, called aquaporin-2 (AQP-2), to the luminal side of the cell membranes. The molecules of AQP-2 cluster together and fuse with the cell membrane by exocytosis to form water channels that permit rapid diffusion of water through the cells. There are other aquaporins, AQP-3 and AQP-4, in the basolateral side of the cell membrane.