Ion channels and the kidney (L1-4) Flashcards
In what ways are ion channels classified?
Classified via selectivity (what ions do they let through?), Gating (what opens and shuts them), and regulation (what regulates the channel?)
Molecular families are based on amino acid sequence and structure.
What is meant by the Nernst potential of an ion? How is it calculated?
When ion channels are open, they drive the membrane potential towards the Nernst (reversal) potential for the channel. Nernst = the conditions when an ion is in equilibrium across a membrane. This is when the voltage difference equals the equilibrium potential (Eion)
Eion = potential net flow of ions
Eion = (RT/vf) x Ln ([ion out]/[ion in])
Explain the Nernst potential of ions at body temp
At body temp - RT/F = 61.5. SO potassium at body temp, Ek = -89mV and Ena = +66mV. When sodium is higher than potassium, it’s about 5x more selective. When potassium is higher than sodium, it’s about 50x more selective. Ecl of chloride is very similar to Ek (-87mV). The fact that K is more selected for, the resting membrane potential ends up being closer to Ek, then when the sodium channels open, the membrane potential rises towards the Ena because those channels are more open.
How do you find the total current carried by a population of channels?
I=N x Po x g x (Vm=Eion)
You can alter N by membrane shuttling or endocytosis of channels. Po can be altered by closing the channels e.g. via phosphorylation, calcium or G proteins. You can change the membrane potential through activation or inhibition of other channels
How can you identify ion channel currents?
You can identify ion channel currents using whole cells patch clamp techniques- clamp to a specific potential and then measure the total current flow across the membrane. You can add a blocker of an ion channel o see if the current decreases. If the current becomes 0 the blocker is blocking the channel completely. NaV channels are closed at negative potentials (therefore their current is 0. They are activated quickly with depolarization, giving an increased current. They then close again with depolarisation
What are the symptoms and probable causes of FHEIG?
Symptoms include bi-temporal narrowing, hypertrichosis (extra hair), this upper lip, bushy/long eyebrows. Delayed development of intellectual ability and motor skills, seizures and EEG anomalies. Its thought to be caused by mutations in K+ channels - Esp. KCNK4. Mutants have larger currents (gain of function) - the mechanism is still unclear. KCNK4 is expressed in the CNS and PNS. In the wt KCNK4, there are low levels of K+ in the interstitial space, and other K+ channels are open. In the mutants, K+ is lost into the interstitial space so conc is high- this increases K+ and causes a change in Ek (makes it more positive). This causes neighbouring cells to have a more depolarised resting potential (means APs are fired more easily)
How does the glomerulus filter blood?
The glomerulus filters blood plasma that passes through the kidney. Water and small molecules have free passage. The passage of blood cells and proteins is restricted because they’re too big. Filters about 180L per day. Total plasma passes the filtration barrier about 65 times a day. The afferent arteriole brings blood into the glomerulus and the efferent takes it out. The glomerulus is surrounded by the Bowman’s capsule and collects any filtrate. The filtrate then slowly flows into the proximal convoluted tubules to begin its journey through the nephron.
Describe the structure of the filtration barrier
Consists of epithelial cells (podocytes), a basement membrane and endothelial cells (within the capillary)
- flat
-large nuclei
-Circular fenestrations (holes between the cells)
The cells are in contact with each other
- Filters blood cells and platelets (stops them getting out of the capillary)
What are the properties of the basement membrane of the Bowman’s capsule?
Continuous (surround glomerular capillaries)
Acts as the main filtration barrier
Has many glycoproteins
Made of things like collagen, laminin and fibronectin
Negatively charged. It filters based on molecular shape, size (mainly) and charge.
Large molecules are not transported and smalled more negatively charged molecules aren’t because they’re repelled by the membrane. The shape is important too as bulky molecules aren’t filtered
What are the properties of podocytes?
Trabecula (big processes coming out of the cell body)
Pedicles (smaller processes coming off the trabeculae) - they act as feet on the capillaries
The Pedicles interdigitate (like intertwining fingers) - so there are still quite big gaps (slit pores) - They don’t really serve a filtering purpose because the slit pores are too big, but their main role is maintenance and phagocytosis of any molecules that aren’t meant to be there (antigens etc)
What determines filtration?
Size, shape and charge. F/P = filtrate to plasma ration - a freely filtered molecule will have the same conc on the filtrate and plasma, so will have a ration of 1. A non-filtered molecule will have no conc in the filtrate so will have a ratio of 0. Therefore F/P ration gives an indication of how likely it is that something is filtered. You can see on a graph that F/p decreases as size increased. And a natural (charged) dextran (chains of glucose) doesn’t filter as much as uncharged dextran
The charge doesn’t really affect very small molecules because they don’t interact with the basement membrane as much (they just move through).
Explain the forces governing glomerular filtration
The filtration coefficient Kf is a constant that gives an idea of the measure of the permeability of a membrane
Starlings forces govern Glomerular Filtration Rate (GFR)
GFR is proportional to the forces favouring filtration - the forces opposing filtration
Forces favouring: Pcap (the hydrostatic pressure in the capillary - pushes plasma out of the capillary, and the oncotic pressure of the Bowman’s capsule (osmotic pressure induced by proteins which are making plasma move into the BC)
Forces opposing filtration are the Pbc (the hydrostatic pressure of the BC, which is opposing movement of plasma out of the capillary) and the oncotic pressure of the capillary, which is trying to draw plasma into it.
Therefore, GFR is proportional to (Pcap+nbc)-(Pbc+ncap)
Along the glomerular capillary - Pbc stays small and constant because fluid enters from glomerulus then flows away. Volume is slightly lower at the end of the capillary. Oncotic pressure increases in capillary because there is a higher conc of proteins as you move along (all the other stuff gets filtered out). n is negligible because its generated from proteins and obvs proteins aren’t in the filtrate (except for a very small amount). So filtration occurs over the length of the whole capillary - unlike in other places where the pressure reverses at the second half. Experimentally, it’s hard to know Kf in a live patient because you have to dissect it out. So you use a clearance technique.
What is the normal GFR, how is it regulated?
Normal GFR is about 125 ml/min, and for a single nephron it’s about 50nl/min.
GFR is maintained at a constant level by autoregulation. BP drops which cause renal blood flow to drop, therefore filtration drops because the hydrostatic pressure decreases, so the nephron adjusts this to increase GFR. A drop if GFR is characteristic of renal failure. GFR is controlled by the afferent arteriole. When renal blood flow increases, autoregulation increases the resistance in the afferent arteriole which decreases the renal blood flow and pressure in the glomerulus. This, therefore, causes a decrease in GFR (think of a car going through a 6 lane toll then driving off into 4 lanes). When arterial BP decreases, causing a decrease in renal blood flow and GFR, autoregulation causes a decrease in resistance, allowing more blood to flow through the glomerulus and increase GFR
What are the 2 theories to how GFR is autoregulated?
- Myogenic theory
- Autoregulation is the property of the afferent arteriole smooth muscle - there are stretch receptors in the smooth muscle itself - Tubuloglomerular feedback theory
- autoregulation controlled by the juxtamedullary apparatus
- Macula densa cells sense the change in the rate of flow via cilia projection. The release vasoactive chemicals which affect the afferent arteriole because they’re close to it. E.g. Increase in GFR and flow, macula densa release vasoconstrictors to decrease blood flow to the glomerulus
What is osmolality?
A measure of how concentrated a solution is
Osmolality = [X] x n in mOSmol/KgH2O
n = number of particles the molecule dissociates into in solution. e.g. 100mM glucose in solution has osmolality of 100 because it doesn’t dissociate into anything, but 100mM of NaCl would be 200 osmolality because it dissociates into 2 molecules (Na and Cl) (theoretically, in reality, things don’t fully dissociate