Effects of chronic growth hormone treatment in aged rats on the biophysical and pharmacological properties of skeletal muscle chloride channels

1. The effects of a 4-month daily treatment with recombinant human growth hormone (GH) (150 μg kg-1) to aged rats were evaluated on the passive and active membrane electrical properties of extensor digitorum longus (EDL) muscle fibres in vitro by means of a two intracellular microelectrode technique. 2. Chronic GH treatment completely restored the diameter and the membrane capacitance of aged EDL muscle fibres and significantly lowered the membrane resistance towards the adult value. There was also an increase of the threshold current, a shortening of the latency and an increase of the amplitude of the action potential and a significant amelioration of the membrane firing capability. 3. The effects were almost fully attributable to a significant 50% increase of resting conductance to chloride ions (G(Cl), although an observed restoration of potassium conductance and a possible effect on voltage-activated sodium channels could contribute to the effects. 4. EDL muscle fibres of untreated aged rats showed a different pharmacological response to 2-(p-chlorophenoxy) propionic acid (CPP) enantiomers from that seen in adult rats; the S-(-) isomer was less potent in blocking G(Cl) and the R-(+) isomer always increased G(Cl) instead of producing the typical biphasic effect observed in adult fibres (an increase of G(Cl) at 1-10 μM and a decrease at higher concentrations). The 4-month-GH-treated aged rats showed a pharmacological sensitivity to CPP enantiomers similar to that of adults. 5. The in vitro application of insulin-like growth factor I (IGF-I), the peripheral mediator of GH, produced a significant and irreversible increase of G(Cl) of EDL muscle of untreated aged rats, an effect not observed in adults. This effect was completely inhibited by preincubation with 0.5 μM okadaic acid, suggesting that the IGF-I receptor transduction pathway can act on the -phosphorylation state of the chloride channel through a serine-threonine protein phosphatase. 6. The results show that the skeletal muscle chloride channel is a target of the impairment of GH/IGF-I axis occurring in aged subjects. The acute and chronic effects observed on G(Cl) of aged muscle fibres suggest that the GH/IGF-I stimuli act through a modulation of channel phosphorylation state and through the synthesis of 'adult'-like type chloride channels.