Experimental modification of rat pituitary growth hormone cell function during and after spaceflight.

Space-flown rats show a number of flight-induced changes in the structure and function of pituitary growth hormone (GH) cells after in vitro postflight testing (W. C. Hymen, R. E. Grindeland, I. Krasnov, I, Victorov, K. Motter, P. Mukherjee, K. Shellenberger, and M. Vasques. J. Appl. Physiol. 73, Suppl.: 151S-157S, 1992). To evaluate the possible effects of microgravity on growth hormone (GH) cells themselves, freshly dispersed rat anterior pituitary gland cells were seeded into vials containing serum +/- microM hydrocortisone (HC) before flight. Five different cell preparations were used: the entire mixed-cell population of various hormone-producing cell types, cells of density < 1.071 g/cm3 (band 1), cells of density > 1.071 g/cm3 (band 2), and cells prepared from either the dorsal or ventral part of the gland. Relative to ground control samples, bioactive GH released from dense cells during flight was reduced in HC-free medium but was increased in HC-containing medium. Band 1 and mixed cells usually showed opposite HC-dependent responses. Release of bioactive GH from ventral flight cells was lower; postflight responses to GH-releasing hormone challenge were reduced, and the cytoplasmic area occupied by GH in the dense cells was greater. Collectively, the data show that the chemistry and cellular makeup of the culture system modifies the response of GH cells to microgravity. As such, these cells offer a system to identify gravisensing mechanisms in secretory cells in future microgravity research.

Skeletal muscle afferent regulation of bioassayable growth hormone in the rat pituitary.

There are forms of growth hormone (GH) in the plasma and pituitary of the rat and in the plasma of humans that are undetected by presently available immunoassays (iGH) but can be measured by bioassay (bGH). Although the regulation of iGH release is well documented, the mechanism(s) of bGH release is unclear. On the basis of changes in bGH and iGH secretion in rats that had been exposed to microgravity conditions, we hypothesized that neural afferents play a role in regulating the release of these hormones. To examine whether bGH secretion can be modulated by afferent input from skeletal muscle, the proximal or distal ends of severed hindlimb fast muscle nerves were stimulated ( approximately 2 times threshold) in anesthetized rats. Plasma bGH increased approximately 250%, and pituitary bGH decreased approximately 60% after proximal nerve trunk stimulation. The bGH response was independent of muscle mass or whether the muscles were flexors or extensors. Distal nerve stimulation had little or no effect on plasma or pituitary bGH. Plasma iGH concentrations were unchanged after proximal nerve stimulation. Although there may be multiple regulatory mechanisms of bGH, the present results demonstrate that the activation of low-threshold afferents from fast skeletal muscles can play a regulatory role in the release of bGH, but not iGH, from the pituitary in anesthetized rats.

Growth hormone, IGF-I, and exercise effects on non-weight-bearing fast muscles of hypophysectomized rats.

The effects of growth hormone (GH) or insulin-like growth factor I (IGF-I) with or without exercise (ladder climbing) in countering the effects of unweighting on fast muscles of hypophysectomized rats during 10 days of hindlimb suspension were determined. Compared with untreated suspended rats, muscle weights were 16-29% larger in GH-treated and 5-15% larger in IGF-I-treated suspended rats. Exercise alone had no effect on muscle weights. Compared with ambulatory control, the medial gastrocnemius weight in suspended, exercised rats was larger after GH treatment and maintained with IGF-I treatment. The combination of GH or IGF-I plus exercise in suspended rats resulted in an increase in size of each predominant fiber type, i.e., types I, I + IIa and IIa + IIx, in the medial gastrocnemius compared with untreated suspended rats. Normal ambulation or exercise during suspension increased the proportion of fibers expressing embryonic myosin heavy chain in hypophysectomized rats. The phenotype of the medial gastrocnemius was minimally affected by GH, IGF-I, and/or exercise. These results show that there is an IGF-I, as well as a GH, and exercise interactive effect in maintaining medial gastrocnemius fiber size in suspended hypophysectomized rats.

IGF-I, growth hormone, and/or exercise effects on non-weight-bearing soleus of hypophysectomized rats.

The effects of insulin-like growth factor (IGF-I) or growth hormone (GH) with and without exercise on predominantly slow muscles of hypophysectomized hindlimb-suspended (HS) rats were determined. HS resulted in a 21, 23, and 30% decrease in soleus, adductor longus, and vastus intermedius masses, respectively, compared with ambulatory rats. Compared with values in HS rats, IGF-I increased the vastus intermedius mass and GH or exercise alone increased both the soleus and vastus intermedius masses. There was a strong interactive effect between GH, but not IGF-I, and exercise in all three muscles of HS rats. The soleus fiber type distribution of HS rats was not affected by any treatment. HS resulted in a 24, 18 (P > 0.05), 32, and 20% (P > 0.05) decrease in the size of soleus fibers containing type I, IIa, I + IIa, and IIa + IIx myosin heavy chains, respectively, compared with ambulatory hypophysectomized rats. Hormone or exercise alone had no effect on fiber size in HS rats. However, all fiber sizes (except for type IIa + IIx in IGF-I with exercise rats) were larger in HS rats treated with GH or IGF-I and exercise than those in HS rats. These data indicate an interactive effect of both GH and IGF-I with exercise in maintaining fiber size of chronically non-weight-bearing predominantly slow muscles. Furthermore, the results suggest that the myosin heavy-chain phenotype in rats deficient in all pituitary factors is unresponsive to short-term administration of either GH or IGF-I or to exercise or HS.

Cognitive impairment in mild traumatic brain injury: a longitudinal diffusional kurtosis and perfusion imaging study.

BACKGROUND AND PURPOSE: Cognitive impairment is frequent among patients with mild traumatic brain injury despite the absence of detectable damage on conventional MR imaging. In this study, the quantitative MR imaging techniques DTI, DKI, and ASL were used to measure changes in the structure and function in the thalamus and WM of patients with MTBI during a short follow-up period, to determine whether these techniques can be used to investigate relationships with cognitive performance and to predict outcome. MATERIALS AND METHODS: Twenty patients with MTBI and 16 controls underwent MR imaging at 3T and a neuropsychological battery designed to yield measures for attention, concentration, executive functioning, memory, learning, and information processing. MK, FA, MD, and CBF were measured in the thalamus by using region-of-interest analysis and in WM by using tract-based spatial statistics. Analyses were performed comparing regional imaging measures of subject groups and the results of testing of their associations with neuropsychological performance. RESULTS: Patients with MTBI exhibited significant differences from controls for DTI, DKI, and ASL measures in the thalamus and various WM regions both within 1 month after injury and >9 months after injury. At baseline, DTI and DKI measures in the thalamus and various WM regions were significantly associated with performance in different neuropsychological domains, and cognitive impairment was significantly associated with MK in the thalamus and FA in optic radiations. CONCLUSIONS: Combined application of DTI, DKI, and ASL to study MTBI might be useful for investigating dynamic changes in the thalamus and WM as well as cognitive impairment during a short follow-up period, though the small number of patients examined did not predict outcome.

Myosin heavy chain composition of adult feline (Felis catus) limb and diaphragm muscles.

The myosin heavy chain (MHC) compositions of adult feline limb and diaphragm muscles were determined. Sodium dodecyl sulfate-polyacrylamide gels (SDS-PAGE) were able to separate three different MHC isoforms. This was in contrast to rat muscles, in which four MHC isoforms were separated by SDS-PAGE. The fastest migrating cat MHC migrated similar to rat type I MHC and labeled in Western blots with a monoclonal antibody (mAb) specific for slow MHC and was categorized as type I. The other two MHC isoforms labeled in Western blots with a mAb specific for fast MHC and were categorized as type II. The slowest migrating fast isoform migrated similar to rat type IIa MHC and labeled with mAb N2.261, specific for types I and IIa; therefore, this MHC was categorized as type IIa. The intermediate migrating cat MHC did not migrate similar to either rat type IIx or type IIb and was not reactive with mAbs N2.261, 35 (specific for rat I, IIa, and IIb MHCs), or F3 (specific for rat IIb MHC). In tissue sections, type IIB fibers (based on myofibrillar ATPase histochemistry) were also unstained with mAbs N2.261 and 35. Therefore, the intermediate migrating cat MHC was categorized as type IIx. Consequently, feline limb and diaphragm muscles were composed of fibers containing type I, IIa, or IIx MHCs. The observations that type I and IIa isoforms, but not IIx, had similar electrophoretic mobilities in the cat and rat and that type IIb was absent from cat limb muscles suggest that there is greater diversity in MHC isoforms IIb and IIx compared to I and IIa in cats compared to rats.

Effects of inactivity on myosin heavy chain composition and size of rat soleus fibers.

Myosin heavy chain (MHC) and fiber size properties of the adult rat soleus were determined after 4-60 days of complete inactivity, i.e., lumbar spinal cord isolation. Soleus atrophy was rapid and progressive, i.e., 25% and 64% decrease in weight and 33% and 75% decrease in fiber size after 4 and 60 days of inactivity, respectively. Changes in MHC occurred at a slower rate than the atrophic response. After 15 days there was de novo expression of type IIx MHC (approximately 10%). By 60 days, type IIx MHC accounted for 33% of the total MHC content, and 7% of the fibers contained only type IIx MHC. The relative amount of type I MHC was reduced from 93% in control to 49% after 60 days of inactivity. Therefore, the effects of 60 days of inactivity suggest that during this time period at least 75% of fiber size and approximately 40% of type I MHC composition of the adult rat soleus can be attributed to activation-related events.

Effect of altered thyroid state on the in situ mechanical properties of adult cat soleus.

To determine the responsiveness of cat hindlimb muscles to thyroid manipulation, adult female cats were made hypothyroid (thyroidectomy plus tapazole treatment), hyperthyroid (synthroid pellets), or maintained euthyroid. After 4 months, the hypothyroid soleus had slower time-to-peak (TPT, 80%) and half-relaxation (HRT) times, whereas the hyperthyroid soleus had faster TPT (20%) and HRT than euthyroid cats. The tension at low stimulation frequencies (5-15 Hz) was higher in hypothyroid and lower in hyperthyroid cats compared to euthyroid cats. Muscle weight, maximum twitch and tetanic (Po) tensions, and maximum rates of shortening (Vmax) were similar across groups. The soleus of hypothyroid cats was more fatigable than normal. The myosin heavy chain (MHC) composition, based on gel electrophoresis, was unaffected by thyroid hormone manipulation. Based on the reaction of monoclonal antibodies for specific MHCs, some fast fibers in the hypothyroid cats coexpressed developmental MHC. These data indicate that 4 months of an altered thyroid state result in changes in the isometric twitch speed properties of the cat soleus, but not the tension-related or isotonic properties. Further, a chronic decrease in thyroid hormone had a greater impact than a chronic increase in thyroid hormone on the mechanical properties of the adult cat soleus.

Interactive effects of growth hormone and exercise on muscle mass in suspended rats.

Measures to attenuate muscle atrophy in rats in response to stimulated microgravity [hindlimb suspension (HS)] have been only partially successful. In the present study, hypophysectomized rats were in HS for 7 days, and the effects of recombinant human growth hormone (GH), exercise (Ex), or GH+Ex on the weights, protein concentrations, and fiber cross-sectional areas (CSAs) of hindlimb muscles were determined. The weights of four extensor muscles, i.e., the soleus (Sol), medial (MG) and lateral (LG) gastrocnemius, and plantaris (Plt), and one adductor, i.e., the adductor longus (AL), were decreased by 10-22% after HS. Fiber CSAs were decreased by 34% in the Sol and by 17% in the MG after HS. In contrast, two flexors, i.e., the tibialis anterior (TA) and extensor digitorum longus (EDL), did not atrophy. In HS rats, GH treatment alone maintained the weights of the fast extensors (MG, LG, Plt) and flexors (TA, EDL) at or above those of control rats. This effect was not observed in the slow extensor (Sol) or AL. Exercise had no significant effect on the weight of any muscle in HS rats. A combination of GH and Ex treatments yielded a significant increase in the weights of the fast extensors and in the CSA of both fast and slow fibers of the MG and significantly increased Sol weight and CSA of the slow fibers of the Sol. The AL was not responsive to either GH or Ex treatments. Protein concentrations of the Sol and MG were higher only in the Sol of Ex and GH + Ex rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Persistence of myosin heavy chain-based fiber types in innervated but silenced rat fast muscle.

Myosin heavy chain (MHC) profile and size of fibers in deep and superficial regions of the adult rat medial gastrocnemius (MG) were determined after 4, 15, 30, and 60 days of inactivity induced by spinal cord isolation (SI). After 4 days, fiber size decreased by 33 to 50% and 36 to 46% in deep and superficial regions, whereas MHC composition was unaffected. By 15 days, these values were 45 to 78% and 51 to 69%, and MHC composition was shifting toward faster isoforms. By 60 days, there were no pure type I MHC fibers and increases from 1 to 18% and 78 to 93% in pure type IIb fibers in deep and superficial regions. The percentage of type I MHC (gel electrophoresis) was approximately 10 and approximately 3%, and of type IIb approximately 40 and approximately 60% in control and 60-day SI rats. Thus, adaptations in the MHC molecule occurred at a slower rate and for a longer duration than the atrophic response.