title: Autonomic regulation of calcium and potassium channels is oppositely modulated by microtubules in cardiac myocytes.
abstract: We recently showed that colchicine treatment of rat ventricular myocytes increases the L-type Ca2+ current (I(Ca)) and intracellular Ca2+ concentration ([Ca2+](i)) transients and interferes with adrenergic signaling. These actions were ascribed to adenylyl cyclase (AC) stimulation after G(s) activation by alpha,beta-tubulin. Colchicine depolymerizes microtubules into alpha,beta-tubulin dimers. This study analyzed muscarinic signals in myocytes with intact or depolymerized microtubules. Myocytes were loaded with the Ca2+ indicator fluo 3 and were field stimulated at 1 Hz or voltage clamped. In untreated cells, carbachol (CCh; 1 microM) induced ACh-activated K(+) current [I(K(ACh))], which happens via betagamma-subunits from the activation of G(i). Carbachol also reduced [Ca2+](i) transients and contractions. Once G(i) is activated by muscarinic agonist, the alpha(i)-subunit is released from the betagamma-subunits, but it is silent, and its inhibition of the AC/cAMP cascade, manifested by I(Ca) reduction, is not seen unless AC has been previously activated. In colchicine-treated cells, CCh caused greater reductions of [Ca2+](i) transients and contractions than in untreated cells. The alpha(i)-subunit became effective in signaling through the AC/cAMP cascade and reduced I(Ca) without changing its voltage-dependence. Isoproterenol (Iso) regained its efficacy and reversed I(Ca) inhibition by CCh. Stimulation of I(Ca) by forskolin persisted in colchicine-treated cells when Iso was ineffective. The effect of CCh on I(K(ACh)) was occluded in colchicine-treated cells. Colchicine treatment, per se, may increase I(K(ACh)) by betagamma-subunits released from G(s) to mask this effect of CCh. Microtubules suppress I(Ca) regulation by alpha(i); their disruption releases restraints that unmask muscarinic inhibition of I(Ca). Summarily, colchicine treatment reverses regulation of ventricular excitation-contraction coupling by autonomic agents.
authors.affiliation: Physiopathologie Cardiovasculaire, Institut National de la Santé et de la Recherche Médicale U-637, EA-3759, Centre Hospitalier Universitaire Arnaud de Villeneuve, F-34295 Montpellier, France. agomez@montp.inserm.fr
authors.forename: Ana M
authors.initials: AM
authors.lastname: Gómez
authors.forename: Benoît-Gilles
authors.initials: BG
authors.lastname: Kerfant
authors.forename: Guy
authors.initials: G
authors.lastname: Vassort
authors.forename: Achilles J
authors.initials: AJ
authors.lastname: Pappano
mesh_terms: D000262:Adenylyl Cyclases / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism; D020746:Calcium Channels, L-Type / Q000378:metabolism*; D002217:Carbachol / Q000494:pharmacology; D018679:Cholinergic Agonists / Q000494:pharmacology; D003078:Colchicine / Q000494:pharmacology; D008297:Male; D008564:Membrane Potentials / Q000502:physiology; D008870:Microtubules / Q000187:drug effects / Q000378:metabolism*; D009200:Myocardial Contraction / Q000502:physiology; D032383:Myocytes, Cardiac / Q000502:physiology*; D015221:Potassium Channels / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D015398:Signal Transduction / Q000502:physiology
chemical_list: D020746:Calcium Channels, L-Type; D018679:Cholinergic Agonists; D015221:Potassium Channels; D002217:Carbachol; D000262:Adenylyl Cyclases; D003078:Colchicine; D002118:Calcium