Regulation of K Balance

Question 1

where is most of the bodies K?

Answer 1

inside cells

Question 2

what is the normal range for plasma K

Answer 2

3.5-5 mM

Question 3

what is the chief determinant of ICFV?

Answer 3

potassium

Question 4

what is responsible for the uneven K distribution in the body?

Answer 4

the Na/K pump- 3 Na out for 2 K in

Question 5

describe the reaction kinetics of the Na/K pump compared to reactant concentrations

Answer 5

the K site is saturated at 1 mM concentrations, much lower than plasma concentrations. however, the Na site is saturated at approximately the internal [Na}, so deviations can alter function

Question 6

what is responsible for MRP

Answer 6

K

Question 7

describe insulins effects on K balance

Answer 7

increases K uptake by cells by 3 mechanisms

1. directly stimulates Na/K pump
2. insulin increases glucose uptake, which is then converted to glucose-6-phosphate. this requires phosphate, which is cotransported with Na, allowing for an uptick in the Na/K pump.
3. activates the Na/H exchanger, which then activates the Na/K pump

Question 8

what treatment may be given to correct hyperkalemia?

Answer 8

insulin with glucose

Question 9

why is there a net intracellular K loss during exercise?

Answer 9

the AP consists of a rapid Na influx and a slower K leak. The K channels are open longer and thus more K is lost than Na can be used to feed Na/K pump.

Question 10

describe the effects of catecholamines on the Na/K pumo

Answer 10

norepinephrine- alters b-receptors, inhibiting the Na/K pump and promoting extracellular K

epinephrine- acts on b-receptors, stimulating the Na/K pump and promoting K uptake

Question 11

how does the body deal with potential K swings during exercise?

Answer 11

it releases epinephrine before to prevent hyperkalemia and norepinephrine after to prevent hypokalemia

Question 12

how does acid-base balance effect K levels?

Answer 12

model: when one K enters, one H leaves

therefore, as cells become K deficient, they become acidic and vice versa

Question 13

how does plasma osmolality affect K balance?

Answer 13

when the plasma is hyperosmotic, it causes cell shrinkage, concentrating [K], and some must leave the cell.

in hypotonic environments, the cells swell, causing a dilution of the K and an uptake.

Question 14

describe K transport in the PT

Answer 14

2/3 of the filtered K is reabsorbed passively, through paracellular pathways caused by solvent drag and positive voltage in the the distal PT

Question 15

describe K transport in the LOH

Answer 15

descending- K is passively secreted d/t high [ ] in medulla

ascending- almost all remaining K, including secretions, is reabsorbed

Question 16

describe K transport in the DT and CCD

Answer 16

cortical collecting duct

K transport here is bidirectional, secretion is the dominant process under normal K intake

principle cells- have the luminal ENaC channels, K secretion is coupled to Na reabsorption. the movement of ions is tied to the Na/K pump, creating a negative voltage luminally and facilitating K secretion

a-intercalated- reabsorb K in exchange for H- H/K-ATPase

Question 17

describe K transport in the MCD

Answer 17

medullary collecting duct

transport is always reabsorption

reabsorbs by both passive and active routes.
active- luminal H/K-ATPase- more superficial
passive- deeper

Question 18

describe the effect of plasma [K] on renal function

Answer 18

2 effects

1. directly stimulates K secretion in DT and CCD=
2. the adrenal gland can sense changes and secretes aldosterone, which causes secretion

Question 19

how does the rate of [na] absorption affect k dynamics?

Answer 19

in the DT and CCD, the rates are coupled together, so an increase in Na reabsorption causes an increase in K secretion

Question 20

aldosterone can be stimulated in 2 ways. how do the effects differ?

Answer 20

1. RAAS- stimulated by low Na, causes increases in Na reabsorption in the proximal tubule, so less Na is seen at the CCD and DT, sparing K
2. adrenal gland- stimulated by elevated K, this does not activate the PT and only the coupled Na/K transport in the CCD and DT responds, resulting in K wasting

Question 21

describe the effect of anion composition on K transport

Answer 21

part of K secretion is mediated by a positive luminal voltage. Cl will also respond to this voltage, leaving the lumen, and lessening the strength. the paracellular pathway is impermeable to other anions. after the consumption of a meal, there is both more K and more anions, so the lumen will be more negative and K secretion will be enhanced

Question 22

describe the effect of flow rate on K

Answer 22

two mechanisms

1. slow flow allows secretion to reach an equilibrium, halting secretion. continuous flow washes away luminal K and increases secretion
2. flow bends cilia on principal cells, stimulating K transport

Question 23

describe the effect of ADH on K transport

Answer 23

because increased ADH decreases flow rate, you’d expect a decreased K secretion as well. ADH also stimulated k secretion to cancel out its flow mediated effect and allow K and water regulation to occur independently

Question 24

how does pH affect K transport in the kidney

Answer 24

secretion is favored under alkaline conditions and inhibited under acidic ones

Question 25

describe how to interpret transtubular K gradient

Answer 25

normal values are within 8-9. if someone is hyperkalemic, they should have an elevated TTKG, indicating a normal kidney response. if it is low, this is inappropriate, and kidney function is impaired.