Thapsigargin depletes calcium stores within the endoplasmic reticulum and decreases the peak EPSPs achieved in associated muscle cells after extended periods of high frequency stimulation

Abstract

The primary intent of this study was to determine the extent that intracellular Ca²⁺ stores have in maintaining muscle cell activity. Our study found intracellular calcium stores within the endoplasmic reticulum of nerve cells integral in the maintenance of adequate neurotransmitter release from presynaptic nerve cells when subjected to high frequency, extended duration stimuli. Thapsigargin was used to deplete internal Ca²⁺ stores of fast-extensor crayfish muscle cells. Two experimental treatments were analyzed, one involving only saline and another involving thapsigargin dissolved in saline. We then subjected samples nerve bundles to rapidly occurring electric stimuli in order to determine the effects of depleted internal Ca²⁺ stores. We applied these stimuli for durations of 10, 15, 30, and 60 seconds. Specimen inundated with thapsigargin showed significantly decreased EPSP values over extended periods of time, whereas there was a less significant effect of thapsigargin over decreased time intervals. This suggests that a significant function of Ca²⁺ stores internal to nerve cells is to maintain an adequate amount of neurotransmitters released even under long periods of continual stress.