Autonomic regulation of calcium and potassium channels is oppositely modulated by microtubules in cardiac myocytes

Am J Physiol Heart Circ Physiol. 2004 Jun;286(6):H2065-71. doi: 10.1152/ajpheart.00933.2003. Epub 2004 Jan 22.

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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Adenylyl Cyclases / metabolism
  • Animals
  • Calcium / metabolism
  • Calcium Channels, L-Type / metabolism*
  • Carbachol / pharmacology
  • Cholinergic Agonists / pharmacology
  • Colchicine / pharmacology
  • Male
  • Membrane Potentials / physiology
  • Microtubules / drug effects
  • Microtubules / metabolism*
  • Myocardial Contraction / physiology
  • Myocytes, Cardiac / physiology*
  • Potassium Channels / metabolism*
  • Rats
  • Rats, Wistar
  • Signal Transduction / physiology

Substances

  • Calcium Channels, L-Type
  • Cholinergic Agonists
  • Potassium Channels
  • Carbachol
  • Adenylyl Cyclases
  • Colchicine
  • Calcium