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Notes: [Auszug] All experiments were performed on acutely decapitate cats in which the carotid arteries were tied off and the vertebral arteries clamped. The dorsal roots were cut from L 3 or L 4 to S 3, and various roots were stimulated to elicit test monosynaptic reflexes in a variety of muscle nerves. For ...

Notes: Summary Ca in desheathed trunks of sciatic nerve ofRana pipiens is completely exchangeable with radioactive45Ca.45Ca distributes in three compartments, one intracellular compartment and two extracellular compartments, which exchange Ca with greatly different rate coefficients. Using impermeable solutes (sucrose, inulin, SO4) and electron microprobe techniques, we have identified the rapidly exchanging extracellular compartment as the interstitial space and the slowly exchanging extracellular compartment, accessible to solutes up to a molecular weight of 5,500, as the mesaxonal gap in the myelin sheath. Ca exists in both extracellular compartments as free Ca and in bound form. After equilibration in solutions containing 1 mM Ca, the contents of the extracellular compartments in mmole/kg wet weight are: 1) Interstitial space: free Ca=0.52, bound Ca=0.19; 2) Myelin region: free Ca=0.12, bound Ca 0.27. The intracellular space contains 0.11 mmole/kg wet weight of Ca. After 4 h of washout,45Ca release represents the Ca flux from the intracellular compartment since it can be inhibited 95% by the combined action of Na and Ca removal and the addition of La. Our results suggest further that45Ca release after this time represents a close approximation of the Ca transport across the axolemmal membrane.45Ca, therefore, represents a suitable tracer probe to investigate Ca compartmentalization and Ca exchange in this tissue.

Notes: Abstract The effects of pH (from pH values 6.50–7.10) on isometric tension development and relaxation were investigated in Triton X-100 “skinned” rat caudal artery. Helically cut skinned strips contracted in 21 μM Ca2+ were studied with respect to maximal isometric tension (Po) and rate of contraction (T0.5C), and following relaxation in 18 nm Ca2+, the rate of relaxation (T0.5R). Acidic pH (pH 6.50) decreased Po to 87% of isometric force obtained at pH 6.90, and increased the rate of contraction as shown by a decrease of T0.5C to 80%. In contrast, T0.5R increased 4.5-fold, indicating that with a change of only 0.40 pH units, relaxation rates were dramatically decreased. pCa-tension curves at pH values 6.50, 6.70, 6.90 and 7.10 indicated no significant shift in half maximal activation (pCa50) between pH 6.50 and 6.70, but a significant (P〈0.01) shift in pCa50 between pH 6.70 ([Ca2+]=0.46 μM) and pH 7.10 ([Ca2+]=0.87 μM). Compared to contractions at pH 6.90, myosin light chain (LC20) phosphorylation at pH 6.50 was significantly greater at 30 and 60 s into contraction but not significantly different at 3–10 min. At both pH 6.50 and 6.90, dephosphorylation was rapid and substantially preceded relaxation; LC20 dephosphorylation and relaxation occurred more rapidly at pH 6.90 than at 6.50. At pH 6.50 and 6.90, relax solutions made with increased Ca2+ buffering capacity showed no effect in enhancing T0.5R, suggesting the difference between relaxation rates was not due to Ca2+ diffusion limitations from the skinned strip. We suggest pH changes can after the contractile and relaxation responses in vascular smooth muscle and these effects may be related to LC20 phosphorylation/dephosphorylation regulatory mechanisms.

Notes: Summary 1. The role of cations in nervous conduction in the central nervous system of the herbivorous insect Carausius morosus was studied with external electrodes. 2. The size of the compound action potential in axons (totally desheathed connectives) depends on the sodium concentration of the bathing medium (Fig. 2), the relationship roughly approximating that predicted by the Nernst formula (Table 4). 3. Sodium-free bathing medium reversibly blocks conduction in axons (totally desheathed connectives) within 1 min (Fig. 2). 4. At low concentrations in the perfusion solution, both tetrodotoxin and procaine reversibly block conduction in axons within a few minutes. 5. Lithium ions replace sodium ions in the maintenance of action potential size in axons for about 10 min, but then action potentials gradually decline. The action potential size is restored when the nerve cord is perfused once again with a medium containing the initial sodium concentration (Fig. 3). 6. External sodium concentrations either higher than 180 mM or lower than 150 mM significantly decrease the viability of axons (Fig. 4); we concluded that the sodium concentration of the extracellular fluid of the nerve cord is probably between 150 and 180 mM. 7. The order of effectiveness of potassium, rubidium and cesium ions in suppressing compound action potentials in axons is K+= Rb+〉Cs+ (Fig. 5). 8. The action potentials in axons perfused with a solution containing 27 mM K+ are reversibly suppressed by approximately 37%; we concluded that the potassium concentration in the extracellular fluid of the ventral nerve cord is less than 27 mM. 9. When the calcium concentration in the external medium is reduced from 7.5 to 1 mM, repetitive firing and progressive conduction blockage are produced after nearly 4 hr in connectives in the absence of a fat-body sheath (Fig. 6). 10. When manganous ions (16 mM) are added to the bathing medium, there is little change in the level of axonal electrical activity (Fig. 7). Since manganous ions have been shown to suppress the increase in membrane conductance to calcium during the so-called calcium spikes of some irritable tissues, we concluded that calcium ions do not contribute significantly to the inward current during the action potential. 11. Axonal viability increases with the external magnesium concentration in the range from 0 to 50 mM Mg++ (Fig. 8). However, the actual magnesium concentration of the extracellular fluid may be between 10 and 25 mM. 12. When axons are perfused with magnesium-free solution, the action potentials are reduced to one-half of their initial size only after 2.36±0.42 hr; we concluded that magnesium ions are not carriers of inward current during action potentials. 13. Comparing our data with published data obtained from other animals, we concluded that nervous conduction in Carausius conforms to the classical membrane theory despite the unusual cationic levels in the hemolymph and that a yet undefined homeostatic mechanism maintains extracellular cationic concentrations at favorable levels in the nerve cord. 14. Viability studies indicate that an active transport mechanism is probably located in the fat-body sheath and that this mechanism probably maintains the extracellular sodium concentration at a high level (Table 2). These studies also indicate that the neural lamella and perineurium form a diffusion barrier which is only slightly permeable to the common cations (Tables 2, 3 and Fig. 6). 15. A model (Fig. 9), explaining the regulation of cationic concentrations in the extracellular fluid of Carausius nerve cord, is proposed from available data.

Notes: Summary 1. To test a previously proposed model for ionic regulation (Weidler and Diecke, 1969), the regulation of sodium ions in the central nervous system of the herbivorous insect Carausius morosus was studied with 22Na. 2. The uptake of 22Na is blocked by 5 mM sodium azide in intact, whole nerve cords (Fig. 2, Table 2). From these findings we concluded that sodium ions are actively absorbed by intact nerve cords. 3. When fat-body sheaths are removed, there is no significant difference between the 22Na uptakes of nerve cords bathed in solution containing 5 mM sodium azide and of those not exposed to azide. Upon comparison of these results with those in Fig. 2 and Table 2, we concluded that the active sodium uptake system is located in the fat-body sheath. 4. The desaturation curve of nerve cords completely saturated with 22Na contains the following three components (Fig. 1): a fast component with a half-time of 1.33 min; an intermediate component with a half-time of 0.39 hr; a slow component with a half-time of approximately 2.5 hr. This three-component system conforms to the proposed model with components probably originating in the fat-body sheath space, the extraneural space and the neural space respectively. 5. The size of the fast component is unaffected by metabolic inhibition (Table 2), and it depends on the external sodium concentration (Table 4); therefore we concluded that this component is outside of the active sodium uptake system. Since the site of active sodium uptake was shown to be in the fat-body sheath (probably in the basement membrane), we concluded that this fast component originates in the fat-body sheath space located outside of the basement membrane. 6. Uptakes of 22Na by intact nerve cords for both the slow and intermediate components are significantly decreased when a metabolic inhibitor is present in the bathing medium (Table 2), but sodium uptake for the intermediate component is increased only slightly and that for the slow component not at all when the external sodium concentration is increased from 15 mM to 180 mM. We concluded from these findings that these two components are located within the boundaries of the active sodium uptake system. 7. The half-time of the intermediate component for intact nerve cords metabolically inhibited during the desaturation period was significantly longer than that of uninhibited nerve cords. This result may be explained by the obliteration of the potential difference across the fat-body sheath by the metabolic inhibitor; we concluded that the intermediate component originates in the extraneural space located between the basement membrane of the fat-body sheath and the neural lamella. 8. Since the origins of the fast and intermediate components have been determined, the only logical anatomical site of origin for the slow component is the neural space (i.e., within the confines of the neural lamella). Results are consistent with this hypothesis. 9. Calculations of mean sodium concentrations from space sizes (determined from photomicrographs) and saturated component sizes were consistent with the model and reported values for similar tissues in other animals. Calculated permeabilities for the fat-body sheath and the neural lamella are consistent with previously published viability studies (see Weidler and Diecke, 1969). We concluded that the present study verifies the significant features of the proposed model with regard to sodium ions.

Notes: Myo-inositol (MI) influx as a function of concentration in rat lens consisted of a saturable component, fit by a rectangular hyperbola, and a linear component which was more distinct at high myo-inositol concentrations suggesting passive diffusion. The hyperbolic component was half-maximally saturated (Kt) at 61.3 μM and had a maximal transport rate (Jmax) of 44.6 μMol/kg wet wt/h. The linear component had an apparent permeability coefficient of 1.44 × 10-6 s-1. Sorbitol, which distributed rapidly in the extracellular space (6.83 ml/100 g wet wt), also appeared to enter the intracellular space with a permeability coefficient of 1.37 × 10-6 s-1, similar to that of myo-inositol. The influx of myo-inositol was critically dependent on the concentration of extracellular sodium consistent with a sodium-myo-inositol contransport. The kinetics of influx activation by sodium suggested an apparent 2:1 coupling ratio for sodium and myo-inositol. When potassium was used as sodium substitute, a significantly stronger influx inhibition was observed than with nondepolarizing sodium substitutes, indicating that myoinositol was driven by the electrochemicl gradient of sodium rather than the chemical gradient only. Reducing the extracellular Na concentration increased the MI concentration at which transport was half-maximally activated, suggesting an ordered binding sequence of Na followed by MI. Myo-inositol influx was competitively inhibited by phlorizin with an inhibitory coefficient (Ki) of 35 μM. Phloretin also was capable of inhibition but with a much lesser efficacy. Myoinositol desaturates from the lens at a rate of 0.00862 h-1. Approximately 19% of the efflux can be inhibited with phlorizin, suggesting that it represents carrier-mediated flux. The phlorizin insensitive flux has a rate of 0.00695 h-1 or 1.93 × 10-6 s-1, similar to the Na-independent passive influx. MI influx is due to a Na-dependent, phlorizin-sensitive active transport while the efflux consists largely of a phlorizin-independent passive leakage. © 1995 Wiley-Liss, Inc.