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| ** Epithelial Na<sup>+</sup> channel | | ** Epithelial Na<sup>+</sup> channel |
| * Also stimulates [[Aldosterone| Aldosterone]] | | * Also stimulates [[Aldosterone| Aldosterone]] |
| + | |
| + | ==Aldosterone== |
| + | |
| + | Aldosterone is a steroid hormone which is secreted from the zona glomerulosa of the adrenal gland. It has a mineralocorticoid activity and is the most important regulator of plasma potassium. When plasma potassium increases increased stimulation of aldosterone occurs directly and as a result of Renin-Angiotensin-Aldosterone System (RAAS). It is also the most important regulator of sodium excretion. |
| + | |
| + | ===Release=== |
| + | |
| + | * Release is stimulated by 3 things |
| + | # Corticotropin (ACTH) |
| + | # Angiotensin 2 |
| + | # K<sup>+</sup> |
| + | * Its release is inhibited by Atrial Natriuretic Peptide |
| + | |
| + | * Most increases in the concentration of aldosterone however can be explained by increases in the [[Renin-Angiotensin-Aldosterone System (RAAS) - Anatomy & Physiology| Renin-Angiotensin-Aldosterone System]] and therefore angiotensin 2 and/or by increases in K<sup>+</sup> concentration |
| + | * Only in severe fluid loss does ACTH significantly stimulate the release of aldosterone |
| + | * ANP is secreted in response to sodium/water loading and therefore inhibits aldosterone secretion |
| + | |
| + | ===Action=== |
| + | |
| + | * Diffuses across the cell membrane - lipophillic (essentially steroidal) |
| + | * Of the principal cells of [[Distal Tubule - Anatomy & Physiology| distal tubule]] and [[Collecting Duct - Anatomy & Physiology| Collecting Duct]] |
| + | * Binds to cytoplasmic receptors |
| + | * Works by altering gene transcription and increases synthesis of proteins |
| + | ** Affects ATP levels |
| + | |
| + | =====Sodium===== |
| + | |
| + | * Affects sodium entry and transport |
| + | * Increases number of apical sodium channels, NaCl co-transporters and Na<sup>+</sup>K<sup>+</sup>ATPase |
| + | * Increases activity of the hydrogen sodium exchanger in the apical membrane |
| + | * Increases membrane permeability |
| + | * Increases sodium pump activity |
| + | * Total quantity of sodium is conserved not the actual plasma concentration |
| + | ** When sodium is reabsorbed water follows it so the volume of the plasma is altered rather than the concentration of sodium changing |
| + | ** [[Renin-Angiotensin-Aldosterone System (RAAS) - Anatomy & Physiology| Angiotensin 2]] and [[Aldosterone]] affect sodium but they also affect ECF volume so only quantity affected not concentration |
| + | ** ADH and thirst response also work together to dilute the ECF if concentrations of sodium are high so although there is more NaCl the actual concentration is not really changed. |
| + | * If there was no secretion of aldosterone a 20kg dog would excrete 15g per 24 hours |
| + | * At maximal secretion no significant amount of sodium would be excreted |
| + | |
| + | =====Potassium===== |
| + | |
| + | * In cases of increased K<sup>+</sup> |
| + | * Increased Na<sup>+</sup> / K<sup>+</sup> ATPase pump activity increases the amount of K<sup>+</sup> in cells to reduce plasma K<sup>+</sup> |
| + | * Generally not excreted |
| + | * However if plasma K<sup>+</sup> is still high aldosterone is stimulated |
| + | * Causes potassium secretion |
| + | ** Stimulates Na<sup>+</sup> / K<sup>+</sup> ATPases in the basolateral membrane of the principal cells |
| + | ** Increased potassium in the cells |
| + | ** Potassium leaves via apical leak channels |
| + | ** Thanks to electro-chemical gradient |
| + | * Very tightly regulated system |
| + | ** Allows large increase in K<sup>+</sup> to have a miniscule effect on plasma K<sup>+</sup> |
| + | |
| + | =====Hydrogen===== |
| + | |
| + | * Increases hydrogen secretion by increasing Hydrogen ATPases in the apical membrane of the intercalated cells |
| + | * Increases hydrogen secretion by increasing sodium hydrogen exchanger in the apical membrane of the principal cells |
| | | |
| <big>'''[[Renin-Angiotensin-Aldosterone System (RAAS) - Anatomy & Physiology| RAAS]]''' | | <big>'''[[Renin-Angiotensin-Aldosterone System (RAAS) - Anatomy & Physiology| RAAS]]''' |