Associated norepinephrine loss following calcium-induced spinal paralysis

A 10% calcium chloride solution or normal physiological saline was instilled onto the intact dorsal surface of the spinal cord via a specially constructed catheter-canopy system fixed above the T9 level in conscious rats. Within 5 min after calcium instillation rats developed flaccid paralysis of the lower limbs with sensory loss. Sensory loss was accompanied by abnormal or negative evoked potentials. Rats instilled with physiological or 10% sodium chloride remained normal. Rats were sacrificed at 1, 16 and 48 h post-calcium exposure and following full functional recovery from paralysis. Spinal cords were removed for histologic and high-performance liquid chromatography (HPLC) analysis. Histologic examination for catecholamines using SPG histofluorescence showed loss of catecholamine-containing varicosities in gray matter below calcium exposure which returned to normal levels upon sensorimotor recovery of hindlimbs about 14 days pce. Light microscopic examination of vascular permeability and general morphology of cord tissue axons and neurons remained normal in calcium and saline instilled rats. HPLC analysis of spinal cord below calcium exposure, also showed norepinephrine (NE) and 3-methoxy-4-hydroxyphenylglycol (MHPG) tissue level reductions which returned to normal upon sensorimotor recovery of paralysis about 2 weeks later. No significant changes were noted in dopamine or serotonin levels in any group. Our findings suggest an impairment of ascending and descending tract transmitter transport, specifically reflected in the noradrenergic bulbospinal pathway. The results implicate a neurofilament-microtubule disassembly in axonal cytoskeleton triggered by the sudden calcium influx

Reconstitution of Cyclin D1-Associated Kinase Activity Drives Terminally Differentiated Cells into the Cell Cycle

Terminal cell differentiation entails definitive withdrawal from the cell cycle. Although most of the cells of an adult mammal are terminally differentiated, the molecular mechanisms preserving the postmitotic state are insufficiently understood. Terminally differentiated skeletal muscle cells, or myotubes, are a prototypic terminally differentiated system. We previously identified a mid-G(1) block preventing myotubes from progressing beyond this point in the cell cycle. In this work, we set out to define the molecular basis of such a block. It is shown here that overexpression of highly active cyclin E and cdk2 in myotubes induces phosphorylation of pRb but cannot reactivate DNA synthesis, underscoring the tightness of cell cycle control in postmitotic cells. In contrast, forced expression of cyclin D1 and wild-type or dominant-negative cdk4 in myotubes restores physiological levels of cdk4 kinase activity, allowing progression through the cell cycle. Such reactivation occurs in myotubes derived from primary, as well as established, C2C12 myoblasts and is accompanied by impairment of muscle-specific gene expression. Other terminally differentiated systems as diverse as adipocytes and nerve cells are similarly reactivated. Thus, the present results indicate that the suppression of cyclin D1-associated kinase activity is of crucial importance for the maintenance of the postmitotic state in widely divergent terminally differentiated cell types