Disuse of rat muscle in vivo reduces protein kinase C activity controlling the sarcolemma chloride conductance

Sabata Pierno¹, Jean-François Desaphy¹, Antonella Liantonio¹, Annamaria De Luca¹, Antonia Zarrilli², Lisa Mastrofrancesco³, Giuseppe Procino³, Giovanna Valenti³, and Diana Conte Camerino¹

¹ Section of Pharmacology, Department of Pharmacobiology, Faculty of Pharmacy, University of Bari, Italy
² Department of Animal Production, Faculty of Veterinary Medicine, University of Bari, Italy
³ Department of General and Environmental Physiology, Faculty of Sciences, University of Bari, Italy

Abstract: Muscle disuse produced by hindlimb unloading (HU) induces severe atrophy and slow-to-fast fibre type transition of the slow-twitch soleus muscle (Sol). After 2 weeks HU, the resting ClC-1 chloride conductance (g_Cl) of sarcolemma, which controls muscle excitability, increases in Sol toward a value typical of the fast-twitch EDL muscle. After 3 days of HU, the g_Cl increases as well before initiation of fibre type transition. Since ClC-1 channels are acutely silenced by PKC-dependent phosphorylation, we studied the modulation of g_Cl by PKC and serine–threonine phosphatase in Sol during HU, using a number of pharmacological tools. We show that a fraction of ClC-1 channels of control Sol are maintained in an inactive state by PKC basal activity, which contributes to the lower g_Cl in control Sol compared to EDL. After 14 days of HU, PKC/phosphatase manipulation produces effects on Sol g_Cl that corroborate the partial slow-to-fast transition. After 3 days of HU, the early increase of g_Cl in Sol is entirely attributable to a reduction of PKC activity and/or activation of phosphatase, maintaining ClC-1 channels in a fully active state. Accordingly, we found that HU reduces expression of PKC, , and isoenzymes in Sol and EDL muscles and reduces total PKC activity. Moreover, we show that the rheobase current is increased in Sol muscle fibres as soon as after 3 days of HU, most probably in relation to the increased g_Cl. In conclusion, Sol muscle disuse is characterized by a rapid reduction of PKC activity, which reduces muscle excitability and is likely to contribute to disuse-induced muscle impairment.

Received for publication 24 July 2007. Revision received 6 September 2007. Accepted for publication 13 September 2007.

Corresponding author D. Conte Camerino: Section of Pharmacology, Department of Pharmacobiology, Faculty of Pharmacy, University of Bari, Via Orabona 4 – campus, 70125 Bari, Italy. Email: conte{at}farmbiol.uniba.it