Rab24 is an atypical member of the Rab GTPase family. Deficient GTPase activity, GDP dissociation inhibitor interaction, and prenylation of Rab24 expressed in cultured cells.

The function of Rab24 is currently unknown, but other members of the Rab GTPase family are known to participate in various protein trafficking pathways. Rab proteins are thought to cycle on and off vesicle membranes in conjunction with changes in their guanine nucleotide state. The present studies indicate that Rab24 possesses several unusual characteristics that distinguish it from other Rab proteins. 1) Based on [(32)P]orthophosphate labeling of protein-bound nucleotide, Rab24 exists predominantly in the GTP state when expressed in cultured cells. The low GTPase activity is related to the presence of serine instead of glutamine at the position cognate to Ras Gln-61. 2) Posttranslational geranylgeranylation of Rab24, determined by metabolic labeling or detergent partitioning assays, is inefficient when compared with other Rabs ending with the common CXC and CC carboxyl-terminal motifs. This is partly due to the presence of two histidines distal to the target cysteines, but also involves other unidentified features. 3) Most of the Rab24 in the cytoplasmic compartment of cultured cells is not associated with Rab GDP dissociation inhibitors. These findings indicate that, if Rab24 functions in vesicular transport processes, it may operate through a novel mechanism that does not depend on GTP hydrolysis or GDP dissociation inhibitor-mediated recycling.

Rat anterior pituitary hormone cells: responses to variable gravity.

BACKGROUND: While life science data dealing with effects of hypogravity are accumulating, relatively little is known about the effects of hypergravity at the level of either the whole animal or the individual cell. The purpose of this experiment was to compare data collected on cells of anterior pituitaries from animals centrifuged at 2G using an experimental design that was identical to that of a spaceflight experiment performed in 1989. HYPOTHESIS: Centrifugation of the animal at 2G for 14 d alters the function and morphology of cells of the anterior pituitary in subsequent in vitro tests at 1G. METHODS: Intact rats were centrifuged at twice Earth's gravity for 14 d in a specially designed animal centrifuge as part of the Cosmos 2G study. This study was designed to replicate a previous spaceflight experiment so that direct comparisons between hyper- (centrifugation, 2G) and hypogravity (spaceflight, 0.001G) could be made. Anterior pituitary cells were then evaluated for cell function and morphology in a variety of post-flight tests. RESULTS: Growth hormone cells from centrifuged animals released less bioactive, but not immunoreactive, growth hormone (GH) than cells from non-centrifuged animals. This was also true for GH released in response to provocative stimulation by a synthetic peptide (growth hormone releasing hormone, GHRH) that causes GH release after binding to cell membrane receptors. Cell morphology was also different between cells from centrifuged and control animals; cells in the experimental group were smaller and less granulated. However, another type of hormone-producing cell contained in these preparations, viz. prolactin cells, was not affected by centrifugation. CONCLUSIONS: Centrifugation of animals for 14 d alters both the in vitro release of GH and GH cell morphology relative to corresponding controls from non-centrifuged animals. Because prolactin cells are not affected by centrifugation, the response is specific to the GH cell.

Relationship of the antispasticity effect of tizanidine to plasma concentration in patients with multiple sclerosis.

BACKGROUND: Spasticity is a serious problem in multiple sclerosis (MS) and many patients do not achieve a satisfactory response to currently available oral antispasticity drugs. Tizanidine hydrochloride, an alpha 2-noradrenergic agonist, has been shown to have an antispasticity effect in single center trials of patients with MS. OBJECTIVE: To compare plasma concentrations of tizanidine with objective measures of muscle tone in patients with MS with moderate to severe spasticity. SETTING: Ten centers, all tertiary referral centers for the specialized treatment of patients with MS, in the United States and Canada. DESIGN: A randomized, double-blind, placebo-controlled, dose-response study of tizanidine hydrochloride (8 or 16 mg). PATIENTS: One hundred forty-two patients with spastic MS who were not taking any interfering medication, such as an antispasticity drug or other alpha-noradrenergic agonist, entered the trial. RESULTS: Tizanidine treatment reduced muscle tone significantly, as shown by improved Ashworth scores and increased knee swing amplitude recorded by the pendulum test, both of which correlated significantly with plasma concentration. Placebo had no significant effect on muscle tone. Dizziness, drowsiness, dry mouth, and fatigue were reported most often in the group treated with tizanidine at peak plasma concentration. CONCLUSIONS: Tizanidine reduces spasticity in MS, and both therapeutic effects and side effects are related to the plasma drug levels.

Efficacy and safety of tizanidine in the treatment of spasticity in patients with spinal cord injury. North American Tizanidine Study Group.

Tizanidine, an imidazoline that acts as an agonist at alpha 2-adrenergic receptors, has been shown to be effective in reducing spasticity caused by MS. This multicenter study (14 sites) assessed the efficacy and safety of oral tizanidine in patients who had spinal cord injury of > 12 months' duration. Of the 124 patients admitted to the study, 78 completed it. Tizanidine was titrated to an optimized dosage in each patient to a maximum of 36 mg/d. Muscle tone, assessed by Ashworth score, was significantly reduced (p = 0.0001) by tizanidine treatment in comparison with placebo. Video motion analysis of the pendulum test showed improvement in the tizanidine-treated patients vs placebo (p = 0.04) and showed a significant correlation with the Ashworth score (p < 0.001). No significant alterations in muscle strength or vital signs were noted in either treatment group. The most common adverse events during tizanidine treatment were somnolence, xerostomia, and fatigue. It was concluded that, overall, tizanidine is effective in reducing spasticity in patients with spinal cord injury.

Pituitary cells in space.

Cells of the mammalian pituitary gland synthesize and secrete several protein hormones which regulate a number of organ systems throughout the body. These include the musculoskeletal, immune, vascular and endocrine systems. Since changes occur in these tissues as a result of spaceflight, and since pituitary growth hormone (GH) and prolactin (PRL) play a role in the control of these systems on earth, we have focused attention over the last 10 years on GH and PRL cell function during and after spaceflight. The cumulative results of 4 spaceflight missions and several mimicked microgravity experiments establish 1) that production and release of biologically active GH and PRL is repeatedly and significantly attenuated (usually > 50%) and 2) that changes in cell morphology also occur. In this paper we describe our results within the framework of methodologies and approaches frequently used to study pituitary cell function on earth. In so doing we hope to develop future flight experiments aimed at uncovering possible microgravity "sensing systems" within the pituitary cell.

Effects of spaceflight on rat pituitary cell function.

The secretory capacity of growth hormone (GH) and prolactin (PRL) cells prepared from rats flown in space on the 12.5-day mission of COSMOS 1887 and the 14-day mission of COSMOS 2044 was evaluated in several postflight tests on Earth. The results showed statistically significant and repeatable decrements in hormone release, especially when biologic (rather than immunologic) assays were used in the tests. Significant and repeatable intracellular changes in GH cells from the flight animals were also found; most important were increases in the GH-specific cytoplasmic staining intensities and cytoplasmic areas occupied by hormone. Tail suspension of rats for 14 days, an established model for mimicking musculoskeletal changes in rats flown in space, resulted in some changes in GH and PRL cell function that were similar to those from animals flown in space. Our results add to a growing body of data that describe deconditioning of physiological systems in spaceflight and provide insights into the time frame that might be required for readaptation of the GH/PRL cell system on return to Earth.

Separation of rat pituitary secretory granules by continuous flow electrophoresis.

The separation of growth hormone-containing cytoplasmic secretory granules from the rat pituitary gland by continuous flow electrophoresis is described. The results are consistent with the hypothesis that granule subpopulations can be separated due to differences in surface charge; these, in turn, may be related to the oligomeric state of the hormone.