Energy Control During Platelet Secretion:Predominant Role Of ATP-Turnover

Platelet aggregation and secretion of granular contents require metabolic energy. This implies the existence of a control mechanism that adjusts the rate of energy producing pathways to the energy need of these functions. Such a control function has been attributed to the level of metabolic ATP, to the adenylate energy charge (AEC =(ATP + 1/2ADP)/(ATP + ADP + AMP) and to the velocity of energy generation and consumption (ATP-turnover). In this study we investigated which factor dominates in control of dense granule (3H-serotonin)-, α-granule (β thromboglobulin)- and lysosomal granule (N acetyl β glucosaminidase) secretion. Human gel- filtered platelets were incubated in glucose-free, CN containing medium. Under these conditions ATP-generation only took place in glycogenolysis, which alone was unable to maintain ATP homeostasis. Consequently, metabolic ATP and AEC fell. Addition of glucose restored glycolytic ATP resynthesis wich restored the AEC but the loss of ATP was for the main part irreversible due to hypoxanthin formation. With this system a broad range of metabolic ATP levels and AEC’s could be obtained both at decreasing ATP turnover (before glucose addition) and at increasing ATP turnover (after glucose addition). Analysis of secretion velocity of dense-, α- and lysosomal granules after initiation with thrombin (5 U/ml) showed that at decreasing turnover the secretion velocity of all types of granules depended on both metabolic ATP level and AEC. In contrast, at increasing ATP turnover the correlation between secretion velocity and metabolic ATP level or AEC was lost. Instead, dense granule secretion was faster and lysosomal granule secretion was slower than expected on the basis of metabolic ATP level or AEC, whereas α-granule secretion showed intermediate levels. The data indicate that the three types of granule secretion differ in their dependence on metabolic energy and that apart from metabolic ATP and AEC, ATP turnover is of crucial importance for secretion of granular contents.