Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Reabsorption and Secretion  ADH  Hormone that causes special water channels to appear in apical cell membranes  Increases rate of osmotic water movement  Higher levels of ADH increase  Number of water channels  Water permeability of DCT and collecting system

Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Reabsorption and Secretion  Without ADH  Water is not reabsorbed  All fluid reaching DCT is lost in urine  Producing large amounts of dilute urine

Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Reabsorption and Secretion Figure 26–15 The Effects of ADH on the DCT and Collecting Duct.

Synthesis of ADH It is synthesized as pre-prohormone and processed into a short peptide (nine amino acids). Six of the amino acids form a ring structure, joined by disulfide bonds. It is very similar in structure to oxytocin, differing only in amino acid #3 and #8. ADH synthesized in the cell bodies of hypothalamic neurons ADH is stored in the posterior pituitary

Actions of ADH The major action of ADH is on renal cells that are responsible for reabsorbing free water from the glomerular filtrate. ADH responsive cells line the distal convoluted tubules and collecting ducts of the renal medulla. ADH increases the permeability of these cells to water. The increase in membrane permeability to water permits back diffusion of water along an osmotic gradient.

Actions of ADH ADH action in the kidney is mediated by its binding to V2 receptors, coupled to adenylate cyclase and cAMP production. cAMP activates protein kinase A which prompts the insertion of water channels into the apical membrane of the cell. When ADH is removed, the water channels withdraw from the membrane and the apical surface of the cell becomes impermeable to water once again.

Pathway by which ADH secretion and tubular permeability to water is increased when plasma volume decreases

Aquaporin proteins embedded in the cell membrane that regulate the flow of water integral proteins can be called “water channels” play a major role in reabsorption in the collecting duct due to ADH

Pathway by which ADH secretion is lowered and water excretion raised when excess water is ingested

Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Reabsorption and Secretion  Aldosterone  Is a hormone produced by suprarenal cortex  Controls ion pump and channels  Stimulates synthesis and incorporation of Na + pumps and channels  In plasma membranes along DCT and collecting duct  Reduces Na + lost in urine

Aldosterone Aldosterone is vital to maintaining sodium and potassium balance and extracellular fluid volume. Aldosterone is an adrenal corticosteroid, synthesized and secreted by the adrenal cortex. Functions mainly is DCT and beginning of CD

The principal function of aldosterone is to sustain extracellular fluid volume by conserving body sodium. Aldosterone is largely secreted in response to signals that arise from the kidney when a reduction in circulating fluid volume is sensed. When body sodium is depleted, the fall in extracellular fluid and plasma volume decreases renal arterial blood flow and pressure. Aldosterone function

Aldosterone action Aldosterone stimulates the active reabsorption of sodium from the tubular urine back into the nearby capillaries in the distal tubule. Water is passively reabsorbed with sodium which maintains sodium concentrations at a constant level. Hence extracellular fluid volume expands in a virtually isotonic fashion.

Increased blood pressure results from excess aldosterone. Hypertension is an indirect consequence of sodium retention and expansion of extracellular fluid volume. Aldosterone action

Renin classified as an enzyme –works similar to a hormone part of the renin-angiotensin system –page 982 helps to regulate the bodies mean arterial blood pressure Angiotensin I  Angiotensin II

Angiotensin II INCREASES BLOOD PRESSURE –constricts blood vessels –increases secretion of ADH –increases secretion of aldosterone –stimulates hypothalamus to activate the “thirst reflex” (acts at the nephron, adrenal glands, and CNS)

ANP and BNP ANP = atrial natriuretic peptide BNP = brain natriuretic peptide Renal effects –increase glomerular filtration –water diuresis –enhanced urinary excretion of sodium Cardiovascular –vasodilation and decrease arterial pressure

Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Reabsorption and Secretion  Natriuretic Peptides (ANP and BNP)  Oppose secretion of aldosterone  And its actions on DCT and collecting system  Parathyroid Hormone and Calcitriol  Circulating levels regulate reabsorption at the DCT