renal lecture 4 Flashcards
what are K+ levels important for?
membrane potentials
what are normal K+ levels?
~4mM, hypokalemia is < 3.5, hyperkalemia is >5
how does K+ homeostasis occur?
balance of input (diet) and output (tissues stores inside cells - rapid, and urine - slower)
where is K+ excretion regulated?
in the distal tubule
how is K+ secretion regulated?
balance between activity of absorption cells (alpha-intercalculated) and secretion cells (principal)
what are alpha-intercalculated cells?
K+ reabsorbtion cells - they have H+K+ATPase in apical membrane and stay consistent
what are principle cells?
K+ secretion cells - they have K+ channels in apical membrane that change depending on plasma concentration of K+ and aldosterone
what happens when K+ plasma concentration increases?
ATPase activity increases so basolateral uptake of K+ increases
this depolarizes the cell and increases the K+ channels in apical membrane
this causes aldosterone release from adrenal cortex
aldosterone increases the ATPase expression, which increases basolateral K+ uptake and increases ENaC channel expression, whciih increases K+ efflux and increases apcial K+ permeability
how is extracellular fluid regulated?
there is an exponential relationship between H+ concentration and pH (more H+ = lower pH/more acidic)
what is plasma pH normally at?
it is buffered to around 7.35-7.45
how is plasma pH kept stable?
it is buffered
what acids effect plasma pH?
acids in from diet (fatty/amino)
acids in from metabolism (CO2, lactic acids, ketoacids)
acids out from expiration (volatile CO2)
acids out from excretion (fixed H+)
what are the two buffers that keep plasma pH stable?
bicarbonate buffer and phosphate buffer
what is the bicarbonate buffer?
CO2 + H2O <-> HCO3- + H+
what is the phosphate buffer?
H2PO4- <-> H+ + HPO42-
which buffer is used most for plasma pH and why?
bicarbonate has more capacity and is easier to regulate and has a lower pKa, it is regulated by lungs and renal, and it is used the most because we have more CO2
we have less phosphate so that is worse
what happens to the products of the carbonate buffer?
H+ is secreted to make H2CO3
it is catalyzed by carbonic anhydrase to make CO2 and H2O, which diffuse into the cell
in cell converted back into H2CO3 (by carbonic anhydrase)
dissasociates into H+ (secreted) and HCO3- (which is reabsorbed using Na cotransporters (NBC) and Cl- counter-transporters (AE)
this all = reabsorbed NaHCO3 and no net secretion of H+ (no pH change)
how is intracellular pH kept stable?
buffered by organic phosphates/proteins
also regulated by membrane transporters/cotransporters/countertransporters
what are beta intercalated cells?
bicarbonate secreting cells (H+ reabsorbing)
whereas alpha-intercalated cells secrete H+ and reabsorb HCO3-
what determines HCO3- reabsorbtion vs secretion?
the relative activity of alpha vs beta intercalated cells
how is H+ secreted in the distal tubule and duct?
combines with CO2 and H2O to make HCO3- which is swapped with Cl- basolaterally
how is H+ excreted as titratable acid (like with phosphate)?
HCO3- is reabsorbed and dissasociates into Co2, H2O, and H+, and H+ is transported by ATPase, and makes H2PO4- (titratable and is excreted)
how is H+ secreted in the proximal tubule?
H+ is secreted as NH4
ammoniagenesis - each glutamine molecule metabolized generates 2 NH4+ (excreted) plus two new HCO3- (absorbed)
how is H+ excreted in the collecting duct?
NH3 diffuses into tubular lumen where it is trapped and excreted as NH4+
