When vesicles of the cardiac sarcoplasmic reticulum (CSR) are incubated with ATP

Question

When vesicles of the cardiac sarcoplasmic reticulum (CSR) are incubated with ATP, Mg2+ and Ca2+, they take up Ca2+ and reach a pseudo steady-state. This is a steady state that changes, but slowly. The uptake of Ca2+ is mediated by the SERCA2a CaATPase. The uptake reaction can be quenched by adding EGTA to the external solution, which binds the Ca2+ outside of the vesicles, or by adding glucose plus hexokinase, that converts the ATP to ADP and glucose-6 phosphate. When the uptake reaction is stopped, Ca2+ that was already taken up by the vesicles leaks out passively.

A. The amount of Ca2+ taken up by the vesicles is generally normalized to the amount of CSR protein in mg rather than being expressed as a concentration. A typical steadystate Ca2+ uptake is 40 nmol mg-1. In separate experiments, the enclosed volume of the CSR vesicles was determined to be 5 µL mg-1. What is the approximate concentration of Ca2+ inside the vesicles at steady-state? The total Ca2+ concentration is its amount divided by the volume in which it is distributed. In this case the amount is 40 nmol mg CSR protein-1 and the volume is 5 µL mg CSR protein-1. The concentration is estimated as

B. The average vesicle size determined by electron microscopy is about 150 nm. What is the volume and surface area of a vesicle this size, assuming it is a sphere?

C. Given that the enclosed volume of the aggregate vesicles is 5 ìL mg-1, how many vesicles are there per mg of CSR protein? How much surface area is there per mg of CSR protein?

D. The initial passive efflux at a load of 40 nmol mg-1 when the pump is stopped is 16 nmol min-1mg-1. Convert this to a flux in units of nmol cm-2 s-1 by dividing by the surface area per mg of CSR protein and converting min to s.

E. What is the passive permeability to Ca2+ in cm s-1?

Explanation / Answer

the amount of ca2+ taken up by the versicles is generally normalized to the amount of csr protein in mg rather than being expreded as a concentration . a typical steady state. ca2+ uptaken is 40 nmol mg 1. in separate experminate the enclosed volume of the crs vesicles was determind to be 5 l mg - 1 what is the approximate concentration of ca2+ the vesicles at steady state of ca2+ concentration is it amount divided by the volume in which it is distribute in case amount is 40 nmol mg csr protein -1 and the volume is mg csr protein the concentration is estimated.

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