Combined response of plasma and platelet catecholamines to different types of short-term stress.

Platelet catecholamines (CAs) do not seem to reflect the acute increases in plasma CAs observed during stress, at least over short periods of time. However, it is well known that stress, via the release of plasma CAs, is capable of affecting platelet function. To better address this issue, we studied platelet and plasma levels of free noradrenaline (NA), adrenaline (A), and dopamine (DA) in 10 healthy subjects undergoing three different stress tests: insulin tolerance test (ITT), cold pressor test (CPT) and handgrip test (HT). We also evaluated platelet count and platelet mean volume (MPV). ITT caused a considerable increase in plasma NA and a massive release of plasma A, while CPT and HT induced discrete increases only in plasma NA. ITT induced a significant decrease in platelet DA and a trend toward a decrease in platelet NA, along with a significant increase in MPV. All these changes occurred in coincidence with the peak levels of plasma CAs. No changes in platelet variables were observed in response to both CPT and HT. The modifications observed during ITT are likely to represent signs of an initial platelet activation in response to the acute plasma CA elevations this test was able to elicit. Our data show that platelet CA content does not represent a mere reflection of the circulating concentrations of plasma CAs, but it is the result of a dynamic balance between these two compartments.