Role of glycogenolytic derived ATP in sarcoplasmic reticulum Ca2+ uptake properties

Introduction:
Glycogen derived energy production affects muscle E-C coupling events as excitability and sarcoplasmic reticulum (SR) Ca²⁺ release (Ørtenblad and Nielsen 2015, Jensen et al. 2020). However, little is known about the role of glycogenolytic derived ATP on SR Ca²⁺ uptake properties.

Materials and Methods:
In order to test the role of glycogenolytic derived ATP on SR uptake properties, mechanically skinned fibres from rat fast twitch muscle were bathed in solutions with constant global [ATP]. The SR Ca²⁺ content was estimated by repeatedly exposure of the fibre to a load solution for 15 to 180 s, followed by estimation of SR Ca²⁺ content (caffeine 30 mM) after each load time. This was estimated controls and after inhibition of glycogen phosphorylase (GP) with two distinct inhibitors (DAB (2mM) or CP-316,819 (10µM)) or glycogen removal by glucoamylase (GA).

Results:
Inhibiting GP caused a pronounced decrease in maximal SR Ca²⁺ contents (CP or DAB, p<0.001), compared to the same fibres prior to inhibition. Same effect was demonstrated in 6 out of 8 GA fibres, however not statistically sign. Functionally, GP inhibition (DAB or CP) reduced Ca²⁺ loading after a fixed loading time (15s) and the time needed to load to a fixed SR concentration.

Conclusions
This study provides direct evidence that glycogenolytic derived ATP is important for normal SR Ca²⁺ uptake rate and maximal content, even in the presence of high global muscle ATP. Further, these results suggest that inhibition of glycogenolysis induces Ca²⁺ leakage from the SR.