Effects of Mlx-8, a phospholipase A2 from Brazilian coralsnake Micrurus lemniscatus venom, on muscarinic acetylcholine receptors in rat hippocampus

Here, we described the presence of a neurotoxin with phospholipase A2 activity isolated from Micrurus lemniscatus venom (Mlx-8) with affinity for muscarinic acetylcholine receptors (mAChRs). Methods: The purification, molecular mass determination, partial amino acid sequencing, phospholipase A2 activity determination, inhibition of the binding of the selective muscarinic ligand [3H]QNB and inhibition of the total [3H]inositol phosphate accumulation in rat hippocampus of the Mlx-8 were determined. Results: Thirty-one fractions were collected from HPLC chromatography, and the Mlx-8 toxin was used in this work. The molecular mass of Mlx-8 is 13.628 Da. Edman degradation yielded the following sequence: NLYQFKNMIQCTNTRSWL-DFADYG-CYCGRGGSGT. The Mlx-8 had phospholipase A2 enzymatic activity. The pKi values were determined for Mlx-8 toxin and the M1 selective muscarinic antagonist pirenzepine in hippocampus membranes via [3H]QNB competition binding assays. The pKi values obtained from the analysis of Mlx-8 and pirenzepine displacement curves were 7.32 ± 0.15, n = 4 and 5.84 ± 0.18, n = 4, respectively. These results indicate that Mlx-8 has affinity for mAChRs. There was no effect on the inhibition ability of the [3H]QNB binding in hippocampus membranes when 1 µM Mlx-8 was incubated with 200 µM DEDA, an inhibitor of phospholipase A2. This suggests that the inhibition of the phospholipase A2 activity of the venom did not alter its ability to bind to displace [3H]QNB binding. In addition, the Mlx-8 toxin caused a blockade of 43.31 ± 8.86%, n = 3 and 97.42 ± 2.02%, n = 3 for 0.1 and 1 µM Mlx-8, respectively, on the total [3H]inositol phosphate content induced by 10 µM carbachol. This suggests that Mlx-8 inhibits the intracellular signaling pathway linked to activation of mAChRs in hippocampus. Conclusion: The results of the present work show, for the first time, that muscarinic receptors are also affected by the Mlx-8 toxin, a muscarinic ligand with phospholipase A2 characteristics, obtained from the venom of the Elapidae snake Micrurus lemniscatus, since this toxin was able to compete with muscarinic ligand [3H]QNB in hippocampus of rats. In addition, Mlx-8 also blocked the accumulation of total [3H]inositol phosphate induced by muscarinic agonist carbachol. Thus, Mlx-8 may be a new pharmacological tool for examining muscarinic cholinergic function.(AU)

Interaction of Pituitary Adenylate Cyclase-Activating Polypeptide and Angiotensin II on Aldosterone Production in Human Adrenocortical H295R Cells / 대한내분비학회지

BACKGROUND: Evidence is accumulating that aldosterone secretion can be regulated in a paracrine and/or an autocrine manner by several neuropeptides locally released within the adrenal gland. Among neuropeptides, pituitary adenylate cyclase-activating polypeptide (PACAP) is present in high concentration in the human adrenal gland. The purpose of this study was to investigate the action of PACAP and the interaction between PACAP and angiotensin II (AII), the main physiologic aldosterone secretagogue, in aldosterone production in human H295R adrenocortical cells. METHODS: H295R cells were incubated with increasing concentrations of PACAP (10(-11)M~10(-7)M) in the absence or presence of 10(-7)M AII. Aldosterone concentration in the supernatant was determined by RIA. Intracellular cAMP content was measured by RIA and total inositol phosphate (IP) production by anion exchange chromatography. Gene expression of CYP11B2 was studied by RT-PCR. RESULTS: In H295R cells, PACAP stimulated aldosterone production in a dose-dependent manner. Incubation of H295R cells with PACAP in the presence of AII significantly increased aldosterone production, compared with that of PACAP alone. PACAP dose-dependently increased cAMP production, but 10(-7)M AII had no effect on either basal or PACAP-stimulated cAMP production. Total IP production was not affected by PACAP, but was increased by 10(-7)M AII; an increase that was not further increased by addition of PACAP. RT-PCR analysis of H295R cells which were exposed to 10-7M PACAP or 10(-7)M AII showed an increase in CYP11B2 transcript signal. Induction of CYP11B2 mRNA expression in response to treatment with both PACAP and AII was significantly more than that resulting from using PACAP alone. CONCLUSION: The present study demonstrates that PACAP exerts a direct stimulatory effect on aldosterone production through induction of CYP11B2 mRNA expression by adenylate cyclase activation as the main intracellular signal pathway in H295R cells. Furthermore, there may be some additive effects between PACAP and AII on aldosterone production.