In skeletal muscle the resting chloride conductance (gCl), sustained by the ClC-1 chloride channel, controls membrane electrical stability as its absence causes myotonia. The gCl is typically higher in fast-twitch than in slow-twitch muscle and is negatively modulated by Protein Kinase C (PKC). Different PKC isoforms are expressed in skeletal muscle, including the PKC-theta. In PKC-theta-null mice (Sun et al., Nature, 2000) we found a significant increase in gCl of slow-twitch soleus muscle with respect to wild-type, being 1876±53 μS/cm2 (n=41) and 1356±37 μS/cm2 (n=19), respectively. A minor 13% increase of gCl was found in the fast extensor digitorum longus (EDL) muscle. Muscle excitability was reduced accordingly. Chelerythrine, a non-specific PKC inhibitor, further increases gCl by 25% in transgenic soleus muscle showing that other PKC isoforms are involved in the control of gCl. Preliminary experiments suggest indeed an up-regulation of the PKC-alpha isoform in these mice. Minor effect of chelerythrine was found in EDL of PKC-theta null mice. Fluvastatin, known to activate PKC (Pierno et al., Br J Pharmacol, 2009) reduced gCl in EDL muscle more than in soleus muscle of transgenic mice confirming that other PKC isoforms contribute to ClC-1 modulation. No modification of ClC-1 expression was found in soleus and EDL muscle of PKC-theta-null mice compared to wild-type. In these mice we also found a significant reduction of resting calcium concentration in both muscle types due to decreased sarcolemmal permeability at rest. Our results indicate that PKC-theta contributes to the regulation of ClC-1 channel differently in the muscle types and that this isoform can also modulate calcium homeostasis likely by interacting with sarcolemma channels. (ASI-OSMA)