Jesica Raingo

Melanocortin 4 receptor constitutive activity inhibits L-type voltage-gated calcium channels in neurons

The melanocortin 4 receptor (MC4R) is a G protein-coupled receptor (GPCR) that is expressed in several brain nuclei playing a crucial role in the regulation of energy balance controlling the homeostasis of the organism. It displays both agonist-evoked and constitutive activity, and moreover, it can couple to different G proteins. Most of the research on MC4R has been focused on agonist-induced activity, while the molecular and cellular basis of MC4R constitutive activity remains scarcely studied. We have previously shown that neuronal N-type voltage-gated calcium channels (CaV2.2) are inhibited by MC4R agonist-dependent activation, while the CaV subtypes that carry L- and P/Q-type current are not. Here, we tested the hypothesis that MC4R constitutive activity can affect CaV, with focus on the channel subtypes that can control transcriptional activity coupled to depolarization (L-type, CaV1.2/1.3) and neurotransmitter release (N- and P/Q-type, CaV2.2 and CaV2.1). We found that MC4R constitutive activity inhibits specifically CaV1.2/1.3 and CaV2.1 subtypes of CaV. We also explored the signaling pathways mediating this inhibition, and thus propose that agonist-dependent and basal MC4R activation modes signal differentially through Gs and Gi/o pathways to impact on different CaV subtypes. In addition, we found that chronic incubation with MC4R endogenous inverse agonist, agouti and agouti-related peptide (AgRP), occludes CaV inhibition in a cell line and in amygdaloid complex cultured neurons as well. Thus, we define new mechanisms of control of the main mediators of depolarization-induced calcium entry into neurons by a GPCR that displays constitutive activity.

Constitutive activity of the Ghrelin receptor reduces surface expression of voltage-gated Ca2+ channels in a CaVβ-dependent manner

ABSTRACT Voltage-gated Ca2+ (CaV) channels couple membrane depolarization to Ca2+ influx, triggering a range of Ca2+-dependent cellular processes. CaV channels are, therefore, crucial in shaping neuronal activity and function, depending on their individual temporal and spatial properties. Furthermore, many neurotransmitters and drugs that act through G protein coupled receptors (GPCRs), modulate neuronal activity by altering the expression, trafficking, or function of CaV channels. GPCR-dependent mechanisms that downregulate CaV channel expression levels are observed in many neurons but are, by comparison, less studied. Here we show that the growth hormone secretagogue receptor type 1a (GHSR), a GPCR, can inhibit the forwarding trafficking of several CaV subtypes, even in the absence of agonist. This constitutive form of GPCR inhibition of CaV channels depends on the presence of a CaVβ subunit. CaVβ subunits displace CaVα1 subunits from the endoplasmic reticulum. The actions of GHSR on CaV channels trafficking suggest a role for this signaling pathway in brain areas that control food intake, reward, and learning and memory. Summary: A new mechanism that modulates the trafficking of several CaV channel subtypes to the plasma membrane through the constitutive activity of the exceptional G-protein-coupled receptor GHSR.

Cardiac and vascular effects of diltiazem, dobutamine and amrinone, drugs used after myocardial revascularization

Hemodynamic care during postoperative management of myocardial revascularization should include vasorelaxing drugs to insure adequate graft and coronary flow, and stimulation of stroke volume to maintain vascular perfusion pressure. We tested the cardiac (inotropic and lusitropic) and vascular (relaxant) effects of diltiazem (0.1 nM to 0.1 mM), dobutamine (10 microM to 10 mM) and amrinone (10 microM to 1 mM) on isolated rat atria and thoracic aorta, and also on isolated human saphenous vein (HSV) and human mammary artery (HMA). Dobutamine produced a maximal positive inotropic effect (+dF/dt max = 29 +/- 7%) at its ED50 for aortic relaxation (88 +/- 7 microM). Conversely, at their ED50 for aortic relaxation diltiazem depressed myocardial contractility and amrinone did not exhibit myocardial effects. In HSV and HMA contracted with 80 mM potassium, diltiazem and dobutamine (but not amrinone) had a vasorelaxant activity similar to that in rat aorta. Norepinephrine-contracted human vessels were significantly more sensitive than potassium-contracted vessels to the relaxant effect of amrinone (ED50 HMA = 15 +/- 5 microM, ED50 HSV = 72 +/- 31 microM, P < 0.05). We conclude that at concentrations still devoid of myocardial effects dobutamine and amrinone are effective dilators in graft segment vessels and rat aorta contracted by membrane depolarization. If the difference between aortic and myocardial tissue still holds in human tissues, at the appropriate concentrations these drugs should be expected to improve cardiac performance while still contributing to the maintenance of graft patency.

Insulin preincubation effects on rat vessel contractile responses: role of the sarcoplasmic reticulum.

In the present work, we studied the possible mechanisms involved in the insulin-induced acceleration of ET1 contractions. We observed a shortening of the half-life needed to achieve maximal developed force (t(1/2)) at 10(-7) M ET1 in rat aortic rings preincubated for 120 min with 3 nM insulin (control 380 +/- 15 s vs. 319 +/- 8 s with insulin, n = 28, p < 0.05). A tyrosine kinase linked receptor was involved in this effect because it was abolished by 30 microM genistein. Endothelium denudation and 10 microM indomethacin treatment did not effect this insulin effect, suggesting its independence of endothelial-derived factors. The effect was still present when the only source of Ca2+ was intracellular (t(1/2) values in the absence of external Ca2+: control 467 +/- 68 s vs. 213 +/- 28 s with insulin, n = 16, p < 0.05), but was blunted if the sarcoplasmic reticulum (SR) Ca2+ source was suppressed by exposure to 10 microM thapsigargin or 10 microM ryanodine. Preincubation with insulin did not potentiate either SR 45Ca2+ uptake or contractions evoked by caffeine-sensitive SR Ca2+ release. Since 30 microM cheleritrine abolished insulin-induced acceleration of ET1 contractions, we propose that the hormone might enhance a signal pathway related to PKC in order to produce a faster Ca2+ release from the SR.