Basal and serotonin or forskolin stimulated sarcolemmal adenylyl cyclase in the skeletal musculature of the intact and ontogenetically serotonin-free male rat. Quantitative differences in enzyme activity between perineal and other skeletal muscles

In muscle cells, cAMP, as intracellular second messenger, plays the major role, together with inositol triphosphate (IP3), in regulating Ca2+-channel activity, hence directly influencing muscle contraction. The cAMP-synthesizing enzyme adenylyl cyclase (AC), on the other hand, is bound to the cell membrane, at the site of receptors. Some of them, mostly β-receptors, are specifically occupied by neurotransmitters, such as, for instance, catecholamines, which can stimulate the enzyme, and consequentely increase the formation of cAMP. Evidences have recently emerged indicating that a crowd of other, not only Ca2+-, but also Na+- and K+-channel types exist, that can normally alter membrane potentials of muscle- or other excitable cells, and can be activated or inhibited by other transmitters, such as, for instance, serotonin (5-HT) or muscarin (M). We have recently observed that not only 5-HT, but also its precursor itself, L-tryptophan, seems to play a non secondary role in skeletal muscle metabolism. Basis of our above quoted observations was the establishment of an original, experimental 5-HT-free animal model (5-HT-f-male rats) chiefly characterized by dwarfism, male sexual immaturity and wasting of the skeletal musculature with unusual alterations of the motor behaviour. In order to investigate if, and eventually how, the ontogenetical absence of serotonin in these rats could have influenced the basic mechanisms of muscle contractility, we undertook the present study by examining: I. The basal expression of cAMP synthesizing AC of sarcolemmal membrane homogenates of hind limb (HLMs) and in sexually dimorphic perineal muscles (PMs) Is) of control intact and 5-HT-f-male rats, in ages of 60, 90 and 120 days. II. The stimulation of muscle membrane bound AC with 5-HT, and Forskolin. Following results arose: 1) Membrane-homogenates generate increasing basal amounts of cAMP throughout growing age in both control and 5-HT-f-rats. In the latter, however, in a significant lower range. 2) PMs membrane homogenates of intact, 120 days old male rats developed significantly (P < 0.001) higher basal amounts of cAMP than those from HLMs. 3) The findings, by contrast, were inverse in incubations of PMs and HLMs of 120 days old 5-HT-f-rats. 4) Stimulation with 5-HT (10-7 M) induced only a slight, but significative (P < 0.01 vs. initial values) increase of cAMP generated from muscle membrane homogenates of intact HLMs and PMs. Any significant difference in responsiveness to stimulation with 5-HT could be observed between HLMs and PMs. 5-HT-stimulation of 5-HT-f-HLMs and -PMs membranes remained fully ineffective. Stimulation of membrane-bound AC with forskolin (starting from 50, mM upwards) was effective in vigorously activating AC bound to membrane homogenates of HLMs and PMs of both control and 5-HT-f-rats. In the latter, however. only the initial concentration of forskolin evoked a significant rise in developing cAMP. Concentrations of forskolin higher than 10 mM failed to induce further increases of cAMP. We infer from these data that skeletal muscles of 5-HT-f-male rats, having a faulty adenylyl cyclase system at the site of the sarcolemmal s-potassium current receptors, are able to synthetize only sporadic and small amounts of cAMP, and that this circumstance may be considered chiefly responsible for the observed alterations in motor behaviour.