G,, Gi and adenylyl cyclase Platelets contain both of the G proteins that are known to regulate cAMP formation by adenylyl cyclase in most mammalian tissues: G, and Gi. G* is present in at least its 45 kDa form. Gio is present in all three of its known forms, with an order of prevalence Giaz)) GiapGi",(93). Traditionally, agents that increase cAMP levels in plateloB, such as PGIz, are described as having receptors that are coupled to G,, while agonists that suppress cAMP formation, such as thrombin and epinephrine, are described as having receptors coupled to Gi (Fig. 1). Relatively litrle information is available about the forms of adenylyl cyclase present in platelots, beyond the absence of type I enzyme (86). Types Itr and VI have been detected by PCR in the megakaryoblastic HEL cell line (46).

Platelet activation is a process that is better understood in macroscopic terms than in molecular terms-although that is changing rapidly. Over the past 10 years research on platelet activation has focused on identrfying cell surface receptors forknown platelet agonists, defining therole of heterotrimeric G proteins in coupling many of these receptors to intracellu-