1 Silverthorn 19-2

2 If we are denied water, we need to excrete less If we drink a lot of water, we need to excrete more (while still excreting the appropriate amounts of various solutes) If water moves only by diffusion, how can this be accomplished?

3 Figure 28-1 Water diuresis in a human after ingestion of 1 liter of water. Note that after water ingestion, urine volume increases and urine osmolarity decreases, causing the excretion of a large volume of dilute urine; however, the total amount of solute excreted by the kidneys remains relatively constant. These responses of the kidneys prevent plasma osmolarity from decreasing markedly during excess water ingestion

4 If water can’t follow the solute, then excess water relative to solute can get excreted X

5 As a result, can excrete ~20L/day of 50 mOsm/L urine This transporter is a Na-K-2Cl transporter, which is very active in the ascending limb of the loop of Henle

Loop of Henle Sodium Reabsorption – thick ascending limb Ascending Thick Limb of the Loop of Henle Epithelial Cell ATP Tubular Lumen (urine) Capillary Lumen (blood) 2K + 3 Na + Na + K+K+ ROMK channel K+K+ K + recycling Na + Ca +2 Mg +2 Paracellular Pathway Cl - Cl -

7 Germann Adding water channels back into the collect duct, would allow some of that water to get reabsorbed, and that’s what ADH (or vasopressin) does or 4

8 A slightly different version of the previous slide:

9 A slightly more complicated version of the previous slide: Brown D, et al, Traffic Mar;10(3): Sensing, signaling and sorting events in kidney epithelial cell physiology.

10 Section of kidney collecting duct triple-immunostained to show AQP2 (green) and AQP4 (red) in vasopressin- sensitive principal cells, and the proton-pumping V-ATPase (blue) in acid-secreting intercalated cells. In the region of the kidney shown here, the inner stripe of the outer medulla, A-IC express V-ATPase apically. In response to systemic acidosis, V-ATPase pumps accumulate in the apical plasma membrane and proton secretion is activated to help excrete the acid load (Bar = 5 μm). Brown D, et al, Traffic Mar;10(3): Sensing, signaling and sorting events in kidney epithelial cell physiology.

11 V-ATPase AQP2 AQP4 PC IC H+H+ H2OH2O Kidney collecting duct

12 ADH is made in the hypothalamus (paraventricular and supraoptic nuclei) and released from the posterior pituitary

13 Hyperosmolarity sensed by osmoreceptors: Central osmoreceptors (hypothalamic) Hepatic portal osmoreceptors ?? Hypovolemia Atrial baroreceptors Hypotension Arterial baroreceptors ?? Role of angiotensin

14 a | MRI images in the horizontal (upper image) and sagittal (lower image) planes, highlighting areas that show a significantly increased blood-oxygen- level-dependent (BOLD) signal under conditions in which thirst was stimulated in a healthy human by infusion of hypertonic saline. The arrows point to increased BOLD signals in the anterior cingulate cortex (ACC; left- hand arrow) and in the area of the lamina terminalis (right-hand arrow) that encompasses the organum vasculosum laminae terminalis (OVLT). b | Plots showing changes in thirst (upper plot) and changes in the BOLD signals in voxels of interest in the ACC (middle plot) and the lamina terminalis (lower plot) of the subject imaged in part a. The values of plasma osmolality shown in the upper plot represent average changes that were observed in a group of subjects that all underwent the same treatment. The traces show that osmoreceptors in the OVLT stay activated as long as plasma osmolality remains elevated, whereas the activation of cortical areas correlates with the sensation of thirst. c | Frequency plots showing examples of changes in firing rate that were detected during extracellular single-unit recordings obtained from three OVLT neurons in superfused explants of mouse hypothalamus. d | A scatter plot showing the changes in firing rate (relative to baseline) that were recorded from many mouse OVLT neurons during the administration of hyperosmotic stimuli of various amplitudes. The data indicate that osmoreceptor neurons in the OVLT encode increases in extracellular fluid osmolality through proportional increases in firing rate. This plot only shows data from osmoresponsive neurons (approximately 60% of the total neuronal population in the OVLT). Part a modified, with permission, from Ref. 27 ©(2003) National Academy of Sciences. Part b modified, with permission, from Ref. 27 © (2003) National Academy of Sciences and Ref.197 © (1999) National Academy of Sciences. Parts c and d reproduced, with permission, from Ref. 89 © (2006) Society for Neuroscience C.W. Bourque. Nature Reviews Neuroscience 9, (July 2008) Central mechanisms of osmosensation and systemic osmoregulation Brain osmoreceptors are neurons that are endowed with an intrinsic ability to detect small changes in ECF osmolality

15 Figure 28-9 Neuroanatomy of the hypothalamus, where antidiuretic hormone (ADH) is synthesized, and the posterior pituitary gland, where ADH is released. Two actions of ADH: Antidiuretic  Action on kidney  Very sensitive (1-15 pM  Action on V2 receptors to cause insertion of aquaporin 2 into epithelial cell members in the collecting ducts Vasopressor  Higher concentrations required than for antidiuresis  Action on V1 receptors in arterioles  (discrepancy between vasocontrictor and vasopressor effects)

16 Diabetes Insipidus Central – problem with ADH synthesis or secretion Nephrogenic – problem with renal response to ADH ~20 L/day of a very hypotonic urine (~ 50 mOsm/L) Insipidus = Latin for lacking taste

17 How can we make a urine that’s more concentrated than 300 mOsm/L? and we can: ~ 1200 mOsm/L !!

18 Germann 18.9

19 The ascending limb of the loop of Henle pumps solute, but is impermeable to water The adjacent descending limb of the loop of Henle is permeable to water but does not transport solute.

20

21 Silverthorn 19-4

22 Germann 18.9

23 Silverthorn 19-10

24 Figure 28-4 Formation of a concentrated urine when antidiuretic hormone (ADH) levels are high. Note that the fluid leaving the loop of Henle is dilute but becomes concentrated as water is absorbed from the distal tubules and collecting tubules. With high ADH levels, the osmolarity of the urine is about the same as the osmolarity of the renal medullary interstitial fluid in the papilla, which is about 1200 mOsm/L. (Numerical values are in milliosmoles per liter.)

25 Figure 28-5 Recirculation of urea absorbed from the medullary collecting duct into the interstitial fluid. This urea diffuses into the thin loop of Henle, and then passes through the distal tubules, and finally passes back into the collecting duct. The recirculation of urea helps to trap urea in the renal medulla and contributes to the hyperosmolarity of the renal medulla. The heavy dark lines, from the thick ascending loop of Henle to the medullary collecting ducts, indicate that these segments are not very permeable to urea. (Numerical values are in milliosmoles per liter of urea during antidiuresis, when large amounts of antidiuretic hormone are present. Percentages of the filtered load of urea that remain in the tubules are indicated in the boxes.)

26 Figure 28-7 Changes in osmolarity of the tubular fluid as it passes through the different tubular segments in the presence of high levels of antidiuretic hormone (ADH) and in the absence of ADH. (Numerical values indicate the approximate volumes in milliliters per minute or in osmolarities in milliosmoles per liter of fluid flowing along the different tubular segments.)

27 Notice that even in the presence of maximal ADH, some water is lost: obligate water loss. Consider that we cannot make a urine that is more concentrated than ~1200 mOsm/L and that there is a certain amount of organic waste that needs to be excreted in the urine, ~600 mOsm per day. Thus, under those conditions, the minimal urine volume would be 0.5 L/day. (Note that the calculations don’t quite work out with the values presented in this figure.)

28 Negative feedback loop controlling plasma osmolality.

29 Negative feedback loop controlling plasma osmolality. What about feedforward regulation?

30 Note the difference in threshold for VP secretion and thirst.

31 Figure Effect of large changes in sodium intake on extracellular fluid sodium concentration in dogs under normal conditions (red line) and after the antidiuretic hormone (ADH) and thirst feedback systems had been blocked (blue line). Note that control of extracellular fluid sodium concentration is poor in the absence of these feedback systems.