We propose to better understand the causes of diarrheal diseases by studying the intracellular regulation of active intestinal electrolyte transport. We will concentrate on the role of Ca++ and calmodulin in regulation of active rabbit ileal electrolyte transport and will determine 1) whether the Na+ absorptive cell is regulated by changes in intracellular Ca++; 2) whether a Na+/Ca++ plasma membrane exchange process is involved as a physiologic regulator of active ion transport; 3) whether intracellular calcium stores are involved in regulation of basal absorption and stimulated secretion; 4) whether Ca++ calmodulin is involved in regulation of basal absorption and stimulated secretion; and 5) whether drugs which alter Ca++ calmodulin-modulated functions in other tissues affect active intestinal ion transport. The goals of this proposal are to understand regulation of basal absorption and stimulated secretion with an ultimate goal of modifying active intestinal transport in such a way as to be useful in therapy of diarrheal diseases. The methodologies to be used will include: 1) Na+ and C1- transport; measurements of active transcellular transport and influx of Na+ and C1- using the Ussing chamber-voltage clamp technique using radioactive tracers; 2) involvement of Ca++: 45Ca++ influx and efflux using radioactive calcium with an extracellular space marker, and calcium content measurements by atomic absorption spectrometry; 3) involvement of cyclic nucleotides: measurement of intestinal content of the cyclic nucleotides cyclic AMP and cyclic GMP and activities of the intestinal mucosal enzymes adenylate cyclase, guanylate cyclase and phosphodiesterase.