Phagocytosis of Candida albicans and superoxide anion Levels in ring dove (Streptopelia risoria) heterophils: effect of melatonin.

We observed in previous studies on avian heterophils that incubation with either physiological or pharmacological concentrations of the neurohormone melatonin increased the phagocytosis of inert particles (latex beads), and also provoked a decline in superoxide anion levels of those phagocytes. In the present study, we wanted to corroborate whether melatonin acts on the oxidative metabolism that accompanies the respiratory burst during phagocytosis by inducing a more effective phagocytic activity at the same time as exerting an antioxidant effect to eliminate and/or scavenge the free radicals left over after the destruction of the foreign material. To this end, we evaluated the ingestion and destruction of Candida albicans (live particles) by ring dove (Streptopelia risoria) heterophils after different times of incubation (30 and 60 min) with physiological concentrations of melatonin (50 pg/ml diurnal and 300 pg/ml nocturnal), as well as with a pharmacological concentration 23 x 106 pg/ml (100 micro m) of the hormone. In parallel, using the same times of incubation, we evaluated the oxidative metabolism by determining the superoxide anion levels (O2-.). The results show that melatonin, at all the times and concentrations studied, increases both the phagocytosis index (number of C. albicans phagocytosed by 100 heterophils) and the candidicide power (percentage of C. albicans killed of those ingested by 100 heterophils). The effect was dose-dependent. With respect to the oxidative metabolism accompanying the digestion and destruction, there was a decline in superoxide anion levels after incubation with all of the concentrations of the hormone studied. The effect was dose-dependent and most pronounced at 60 min. These results thus corroborate the proposal that melatonin enhances the phagocytic function at the same time as neutralizing the oxidative stress derived from this immune function.