Yessotoxin (YTX) is a novel phycotoxin with an unknown mechanism of action that has been reported as cardiotoxic, when injected, but non-toxic if ingested orally. In this paper, we studied the effect of YTX on adenosine 3',5'-cyclic monophosphate (cAMP) pathway, since this pathway can be a cellular target to this toxin as happens in other diarrhetic toxins. We determined cAMP levels by enzymeimmunoassay and by using the cAMP dye recombinant fluorescein- and rhodamine-labeled protein kinase A, which increases their fluorescence when cAMP levels are increased. In the presence of YTX, and after a transient small increase, cAMP levels were decreased. This effect was Ca(2+) dependent since in a Ca(2+)-free medium YTX increased cAMP levels, but this event was reverted after addition of external calcium. YTX also reverted the increase of cAMP induced by the adenylyl cyclase activator forskolin. These variations in fluorescence units were confirmed when cAMP levels were measured by enzymeimmunoassay, YTX decreases cAMP from 52.81+/-3.66 to 44.53+/-4.5 fmol. Phosphodiesterase (PDE) IV inhibitors, rolipram or etazolate, did not modify the effect of YTX, however, when PDE IV was first inhibited no effect of YTX was observed. On the other hand, the PDE III inhibitor milrinone counteracted the effect of YTX, and a similar effect was observed with the unspecific PDE I inhibitor chlorpromazine. These results point to an effect of YTX on PDE activity. In the presence of YTX, the fluorescent PDE substrate Mant-cAMP, increased its rate of hydrolysis, the same as the PDE from bovine brain increased the hydrolysis of cAMP substrate. In addition, YTX increased interleukin-2 production, which indirectly confirms a decrease in cAMP. Although results show a very complex pattern of responses, due to the interactions and crosstalks between many systems, results suggest that YTX is a PDE activator in the presence of external Ca(2+).