Stimulatory effects of adenosine on prolactin secretion in the pituitary gland of the rat

We investigated the effects of adenosine on prolactin (PRL) secretion from rat anterior pituitaries incubated in vitro. The administration of 5-N- methylcarboxamidoadenosine (MECA), an analog agonist that preferentially activates A2 receptors, induced a dose-dependent (1 nM to 1 µM) increase in the levels of PRL released, an effect abolished by 1,3-dipropyl-7-methylxanthine, an antagonist of A2 adenosine receptors. In addition, the basal levels of PRL secretion were decreased by the blockade of cyclooxygenase or lipoxygenase pathways, with indomethacin and nordihydroguaiaretic acid (NDGA), respectively. The stimulatory effects of MECA on PRL secretion persisted even after the addition of indomethacin, but not of NDGA, to the medium. MECA was unable to stimulate PRL secretion in the presence of dopamine, the strongest inhibitor of PRL release that works by inducing a decrease in adenylyl cyclase activity. Furthermore, the addition of adenosine (10 nM) mimicked the effects of MECA on PRL secretion, an effect that persisted regardless of the presence of LiCl (5 mM). The basal secretion of PRL was significatively reduced by LiCl, and restored by the concomitant addition of both LiCl and myo-inositol. These results indicate that PRL secretion is under a multifactorial regulatory mechanism, with the participation of different enzymes, including adenylyl cyclase, inositol-1-phosphatase, cyclooxygenase, and lipoxygenase. However, the increase in PRL secretion observed in the lactotroph in response to A2 adenosine receptor activation probably was mediated by mechanisms involving regulation of adenylyl cyclase, independent of membrane phosphoinositide synthesis or cyclooxygenase activity and partially dependent on lipoxygenase arachidonic acid-derived substances.

Population structure and relative growth of the Amazon shrimp Macrobrachium amazonicum (Heller, 1862) (Decapoda: Palaemonidae) on two islands in the fluvial-estuarine plain of the Brazilian Amazon

This study aimed to describe the population structure of the Amazon shrimp Macrobrachium amazonicum, as well as their relative growth between the length of the cephalothorax and the total length, and between the length of the cephalothorax and the total mass of shrimps of a fluvial-estuarine plain in the State of Pará. Shrimps were sampled monthly from August 2006 to July 2007, using trawl nets, taking three replicates at each site (Arapiranga and Mosqueiro) per month, totaling 72 replicates. We caught 5,510 specimens, being 90.90% from Arapiranga Island and 9.1% from Mosqueiro Island. The highest densities occurred in July (1.33 individuals/m²), at the beginning of the dry season and in December (1.66 individuals/m²), at the beginning of the rainy season. The morphometric analysis for separate and grouped sexes resulted in negative and positive allometric growth. Ovigerous females were observed in all months, indicating continuous reproduction and the majority (67.81%) was caught during the less rainy season. The abundance and continuous reproduction of M. amazonicum show that this estuary offers conditions for the proper development of this population.