Adenosine is released by hypoxic lung tissue and by IgE-stimulated mast cells. Inhaled adenosine induces bronchoconstriction in asthmatic subjects, and exogenous adenosine markedly potentiates the release of mast cell granule-associated mediators. This mediator release augmentation may be associated with an adenosine-induced increase in mast cell cyclic AMP content, but mast cell adenosine receptors do not fit the criteria for any specific adenosine receptor subtype, and the mechanism of action of adenosine in mast cell secretion in uncertain. Cell surface hormone receptors often interact with specific nucleotide binding proteins that subsequently activate a catalytic unit of adenylate cyclase or other biochemical processes. Whether the mechanism of action of adenosine on mast cells is dependent on N protein coupling and what type of N protein is involved are as yet unclear. The purpose of the studies proposed herein is to examine the mast cell adenosine receptor, its interactions with N proteins, and its functional requirement, if any, for a linkage to cyclic AMP. These questions will be addressed by utilizing agents that selectively activate or block certain components of the receptor - N protein - catalytic unit system, such as cholera toxin, forskolin, or phorbol esters. After considering the nature of the coupling of adenosine receptors to cyclic AMP, the effects of adenosine on inositol phosphate metabolism, calcium mobilization, and protein kinase activation will be explored. The regulation of adenosine receptors by pharmacologic agents will be examined. The ability of aminophylline to up-regulate mast cell adenosine receptors has been described, and the adenosine analog NECA can desensitize mast cell receptors to the subsequent actions of adenosine. Corticosteroids can reverse or inhibit the desensitization of beta-adrenergic receptors in some cell types, but their action on adenosine receptors has not been studied. Agents that alter adenosine metabolism such as 5-deoxy- 5-iodotubericidin an adenosine kinase inhibitor, will also be studied as to their effects on adenosine receptor expression. In general, the proposed project is intended to broaden the understanding of the importance of adenosine receptors in mast cell biochemistry, their interactions with N proteins and their pharmacologic regulation. As adenosine is a bronchoconstrictor and a potentiator of immediate hypersensitivity reactions, information regarding its role in mast cell function should aid in the development of agents useful in the treatment of asthma and allergic diseases.
