Membrane-associated signal transduction modulates contractile responses to Ca2+ in saponin-skinned coronary smooth muscle

Insulin Causes [Ca 2+ ] i -Dependent and [Ca 2+ ] i -Independent Positive Inotropic Effects in Failing Human Myocardium

Background— Insulin has been shown to exert positive inotropic effects in several in vitro and in vivo models, but signal transduction and substrate dependency remain unclear. We examined inotropic responses and signal transduction mechanisms of insulin in human myocardium.

Methods and Results— Experiments were performed in isolated trabeculae from end-stage failing hearts of 58 nondiabetic and 3 diabetic patients undergoing heart transplantation. The effect of insulin (0.3 and 3 IU/L) on isometric twitch force (37°C, 1 Hz) was tested in the presence of glucose or pyruvate as energetic substrate. Furthermore, intracellular Ca 2+ transients (aequorin method), sarcoplasmic reticulum (SR) Ca 2+ content (rapid cooling contractures), and myofilament Ca 2+ sensitivity (semiskinned fibers) were assessed. In addition, potential signaling pathways were tested by blocking glycolysis, PI-3-kinase, protein kinase C, diacylglycerol kinase, insulin-like growth factor-1 receptors, or transsarcolemmal Ca 2+ entry via the Na + /Ca 2+ exchanger. Insulin exerted concentration-dependent and partially substrate-dependent positive inotropic effects. The phosphatidylinositol-3-kinase inhibitor wortmannin and the Na 2+ /Ca 2+ exchanger reverse-mode inhibitor KB-R7943 completely or partially prevented the functional effects of insulin. In contrast, insulin-like growth factor-1 receptor blockade, protein kinase C inhibition, and diacylglycerol kinase blockade were without effect. The inotropic response was associated with increases in intracellular Ca 2+ transients, SR Ca 2+ content, and increased myofilament Ca 2+ sensitivity.

Conclusions— Insulin exerts Ca 2+ -dependent and -independent positive inotropic effects through a phosphatidylinositol-3-kinase–dependent pathway in failing human myocardium. The increased [Ca 2+ ] i originates at least in part from enhanced reverse-mode Na + /Ca 2+ exchange and consequently increased SR-Ca 2+ load. These nongenomic functional effects of insulin may be of clinical relevance, eg, during insulin-glucose-potassium infusions.

Receptor-dependent G protein-mediated Ca2+ sensitization in canine airway smooth muscle

To determine the mechanisms of receptor-dependent Ca2+ sensitization in airway smooth muscle, canine tracheal smooth muscle (CTSM) was permeabilized with alpha-toxin or beta-escin. Although the effects of 5-hydroxytryptamine (100 microM), histamine (100 microM), and the thromboxane A2 analogue U-46619 (100 microM) were negligible, carbachol (100 microM) and endothelin-1 (ET-1, 1 microM) evoked additional contractions of 47.0 +/- 5.90% and 25.0 +/- 5.37% (n = 6) at pCa 6.7 with GTP (3 microM) (normalized to the maximum contraction at pCa 4.5) in alpha-toxin-permeabilized CTSM. GDP-beta-S (1 mM) reversed the carbachol and ET-1 responses completely. GTP-gamma-S (30 microM) and 4 beta-phorbol 12,13-dibutyrate (PDBu, 3 microM) increased the Ca2+ sensitivity (median effective pCa) of contraction by 1.8- and 4.4-fold, respectively (n = 4-11, P < 0.05). The effects of saturating concentrations of GTP-gamma-S and PDBu were additive. A synthetic peptide (T2) corresponding to the actin-binding site of calponin caused a dose-dependent contraction of beta-escin permeabilized CTSM, with the peak effect (25 +/- 4%, n = 4) at 1200 microM, PDBu (3 microM) caused contraction of the T2 peptide-treated CTSM. In conclusion, Ca2+ sensitization of CTSM depends on receptor type and is mediated by G proteins and protein kinase C whose effects are additive, with a partial contribution by calponin.

Effects of GTP gamma S on muscarinic receptor-stimulated inositol phospholipid hydrolysis in permeabilized smooth muscle from the small intestine

1. Smooth muscle fragments from the longitudinal layer of the small intestine of the guinea-pig were permeabilized with Staphylococcus aureus alpha toxin (alpha-toxin) and used to investigate the role of G-protein activation in the regulation of muscarinic acetylcholine receptor (AChR)-stimulated inositol phospholipid hydrolysis. 2. The efficiency of alpha-toxin permeabilization was estimated by the release of [3H]-2-deoxyglucose ([3H]-2DG) after prior loading or lactate dehydrogenase (LDH) enzyme release from the smooth muscle fragments. 3. In alpha-toxin-permeabilized smooth muscle, but not in non-permeabilized muscle, GTP gamma S induced time- and concentration-dependent increases in labelled inositol phosphates. Carbachol (CCh) increased labelled inositol phosphates in both permeabilized and non-permeabilized muscle, although the increases were greater in non-permeabilized smooth muscle. The response to 100 microM CCh was severely reduced by 0.5 microM atropine. 4. In permeabilized muscle the effects of GTP gamma S or CCh on inositol phosphate levels were reduced by treatment with pertussis toxin (PTX) and completely inhibited by GDP beta S. 5. GTP gamma S caused a concentration-dependent inhibition of the CCh-induced increases in the levels of labelled inositol phosphates. Dibutyryl cyclic AMP or Sp-cAMPs (adenosine-3',5'-cyclic phosphorothiolate-Sp) reduced the effects of CCh on inositol phosphate levels. 6. The results suggest that muscarinic AChR activation induces inositol phospholipid hydrolysis via more than one G-protein in this smooth muscle and that several mechanisms may contribute to the modulation of both stimulatory and inhibitory responses observed.

Guanosine 5'-O-(3-thiotriphosphate) causes endothelium-dependent, pertussis toxin-sensitive relaxations in porcine coronary arteries

To determine whether direct stimulation of endothelial G-proteins causes relaxations of the underlying vascular smooth muscle, the effects of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) and sodium fluoride were studied in porcine coronary arteries and endothelial cells. Isometric tension was measured in coronary rings contracted with prostaglandin F2 alpha. GTP gamma S (in the presence of saponin) and sodium fluoride (in the presence of AlCl3) relaxed rings with, but not those without endothelium. The responses were inhibited by nitro-L-arginine and pertussis toxin. In membrane fractions of coronary endothelial cells, GTP gamma S and sodium fluoride inhibited the ADP-ribosylation of G-proteins catalyzed with [32P]-NAD and pertussis toxin. These data suggest that direct stimulation of G-proteins in endothelial cells by GTP gamma S and sodium fluoride causes a pertussis toxin-sensitive relaxation which may be attributed to the release of nitric oxide.

Aluminum fluoride induces a reversible Ca2+ sensitization in alpha-toxin-permeabilized vascular smooth muscle

The mechanism of aluminumfloride (AlF)-induced Ca2+ sensitization was explored in alpha-toxin-permeabilized rabbit mesenteric artery. In the presence of 0.18 microM Ca2+ and deferoxamine, a strong chelator of aluminum (Al3+), fluoride (F-; applied in the form of NaF) induced very slow tension development, while in the presence of tracer levels of Al3+ tension developed rapidly possibly due to formation of Al-F complexes (especially AlF4-). As a result, AlF significantly shifted the relationship between tension development and free Ca2+ concentration in the Ca(2+)-EGTA buffer (pCa-tension curve) to the left. The rate of the tension development also depended on the EGTA concentration: increasing the EGTA concentration from 0.5 to 10 mM markedly decreased the maximal rate of contraction ((dT/dt)max), probably due to chelation of Al3+ by EGTA, without effect on the maximal tension (delta Tmax). The AlF-induced Ca2+ sensitization could be reversed by extensive washing with relaxing solution (pCa greater than 8), in contrast to the contractions induced by guanosine 5'-[gamma-thio]triphosphate (GTP gamma s; a non-hydrolyzable GTP analogue) or phorbol 12,13-dibutyrate (PDBu) which were irreversible. However, the action of all the compounds appeared to be mediated through a H-7 (1-[5-isoquinolinesulfonyl]-2-methylpiperazine dihydrochloride)-sensitive pathway, and no additive effects among them were observed. In addition, GDP increased (dT/dt)max due to AlF without changing delta Tmax, whereas guanosine 5'-[beta-thio]diphosphate (GDP beta s; a non-hydolyzable GDP analogue) decreased both parameters. These findings suggest that AlF acts on G-proteins to enhance Ca2+ sensitivity of contractile elements through a H-7-sensitive pathway.