Medical Physiology 2006

Problem Set 1:  Cellular Physiology I:  Osmotic water transport/cell volume regulation:

 

 

Part I:

 

All adults over the age of 50 should be screened for colon cancer, with the recommended outpatient procedure being colonoscopy.  Prior to the procedure, it is important to flush the colon of all contents.  This is started the day before by having the patient consume a solution that produces copious watery diarrhea.  A typical solution is Fleet Phospho Soda®; the package lists the contents of the solution:

 

“Each teaspoon (5 mL) of Fleet Phospho Soda contains 2.4 grams of monobasic sodium phosphate, and 0.90 grams of dibasic sodium phosphate.”

 

The dosage of the solution necessary to clean out the colon is 3 tablespoons (9 teaspoons = 45 mL) of the solution taken orally with a glass of water.  Note that monobasic sodium phosphate is NaH2PO4 (molecular weight 120) and dibasic sodium phosphate is Na2HPO4 (molecular weight 142).

 

 

1.   What are the molar concentrations (moles per liter) of the two salts in the solution?

 

 

2.   How many grams of sodium are consumed when drinking the three tablespoons (45 mL) of the solution?   (Note that less than 2 grams of sodium daily is the dietary restriction in patients who have heart disease.)

 

Fortunately, only negligible amounts of the phosphate are absorbed into the body by the intestinal tract, and little of the sodium is absorbed as well.  Thus, the sodium and phosphate remain within the lumen of the intestines and colon.  The intestines secrete enough water (from body fluids) into the lumen in order to render the lumen osmolarity equal to that of the other body fluids (namely 300 mOsm).  This water causes the diarrhea that subsequently flushes out the colon.

 

3.   How many osmoles (total moles of osmotic particles) are consumed when drinking the three tablespoons of the solution?

 

 

4.   How much water is secreted into the intestinal lumen to dilute the sodium and phosphate to normal body osmolarity?  (Ignore the volume of water ingested with the solution.  Also ignore the increase in osmolarity of the body fluids resulting from the secretion into the intestines.)

 

 

Part II:

 

A resting neuron is at steady state—its membrane potential is stable, and its internal composition is not changing.  The membrane potential (Vm) is -83 mV, and the intracellular Na+ and K+ concentrations are 10 mM and 140 mM, respectively, when bathed in extracellular solution containing 140 mM Na+ and 4 mM K+.

 

5.   What are the Na+ and K+ equilibrium potentials (ENa and EK, respectively)?  (Note:  answers to the nearest mV are sufficient.)

 

Electrophysiological (e.g., patch-clamp) studies determine that the Na+ and K+ membrane conductances (gNa and gK) are 0.10 mS/cm2 and 1.0 mS/cm2, respectively.

 

6.   What are the membrane currents for Na+ and K+ (iNa and iK) for a unit area of membrane?  (Note:  answers to the nearest mA/cm2 are sufficient.)

 

 

7.   What is the net current carried by the Na+,K+-ATPase (sodium pump)?  (Note:  pump stoichiometry is 3:2, namely 3 Na+ pumped out in exchange for 2 K+ pumped in.)

 

 

It is also known a priori that the total cell-membrane conductance is accounted for by gNa, gK and gCl (i.e., the membrane has Cl- channels as well), but gCl is not known.  Also, it is known that Cl- enters or exits the cell solely via the passive Cl- channels (e.g., there are no Cl- pumps).  The extracellular solution has a Cl- concentration of 105 mM.

 

8.   What is the Cl- current (iCl) flowing across the membrane?

 

 

9.   What is the intracellular Cl- concentration?

 

 

 

 

Part III:

 

A neuron cell body exhibits the following:  ENa = +50 mV, EK = -100 mV and ECl = -75 mV.  Under resting conditions, the membrane conductances are as follows:  gNa = 20 nS, gK = 100 nS and gCl = 0 nS.  When action potentials arrive at an excitatory synapse on the cell, an EPSP is generated resulting from an increase in gNa and gK of 10 nS each.  Similarly, when an action potential arrives at an inhibitory synapse on the cell, an IPSP is generated resulting from an increase in gCl of 20 nS. 

 


10.    Fill in the entries in the following table, where EPSP refers to the time when the excitatory stimulus is occurring, IPSP refers to the time when the inhibitory stimulus is occurring, and EPSP+IPSP refers to the time when both are occurring simultaneously (e.g., spatial summation).  (The effects are subtle, so compute Vm to an accuracy of ±0.1 mV.)  Be able to explain the results you obtain!