Effect of monovalent-divalent cation exchange on the swelling of polyacrylate hydrogels in physiological salt solutions.

The volume transition induced by monovalent-divalent cation exchange of fully neutralized polyacrylate hydrogels was investigated in aqueous NaCl solutions. The variation of the osmotic swelling pressure, shear modulus, and mixing pressure was measured when Na(+) ions were substituted by divalent or trivalent cations. Alkali metal salts move freely throughout the entirely network, and alkaline earth metal salts (CaCl(2), SrCl(2)) promote aggregation of polyacrylate chains, but these aggregates are relatively weak. Transition metal salts (CoCl(2), NiCl(2)) form stronger interchain associates. Rare earth cations (La(3+) and Ce(3+)) bind practically irreversibly to the polymer. Experimental data indicate that transition metal cations modify both the elastic and mixing components of the free energy, while alkaline earth metal cations affect primarily the mixing term. The behavior of freely swollen gels was compared with similar gels subjected to uniaxial compression. In uniaxially compressed gels, volume transition occurs at lower cation concentrations than in the corresponding undeformed gels. The shift of the transition point increases with the deformation ratio and is larger for Co(2+) than for Ca(2+).

Osmotic swelling of polyacrylate hydrogels in physiological salt solutions.

The swelling behavior of fully neutralized sodium polyacrylate gels was investigated in aqueous solutions of alkali metal (LiCl, NaCl, KCl, CsCl) and alkaline earth metal salts (CaCl2, SrCl2, BaCl2). The total salt concentration and the ratio of monovalent to divalent cations were varied in the biologically significant range. It is found that the concentrations of both monovalent and divalent cations vary continuously and smoothly in the gel despite the abrupt change in the gel volume. The individual elastic, mixing, and ionic contributions to the free energy of the gel were separately determined as a function of the degree of network swelling to elucidate the thermodynamics of swelling. Shear modulus measurements performed at different Ca2+ concentrations suggest that Ca2+ does not form stable cross-links between the polymer chains. At low and moderate swelling ratios the concentration dependence of the shear modulus follows a power law behavior, G variation of phi n, with n = 0.34 +/- 0.03. At high swelling degrees, however, the shear modulus increases with increasing swelling. The value of the Flory-Huggins interaction parameter, chi, determined from osmotic swelling pressure and shear modulus measurements, strongly depends on the ionic composition of the equilibrium solution and increases with increasing Ca2+ concentration.

Rapid structural changes in nerve fibers and cells associated with their excitation processes.

In a variety of nerve fibers, cells and other excitable tissues, the electric responses to electric stimuli were found to be accompanied by a transient swelling of the tissues. The rising phase of this swelling coincides with that of the electric response. By use of a heat-sensor made of polyvinylidene fluoride film, it was also shown that the electric responses of many types of excitable tissues are accompanied by simultaneous heat production. To elucidate the origin of this swelling and heat production, the process of Ca2+-Na+ exchange in synthetic anionic gel beads and rods was investigated. It is asserted that the observed signs of nerve excitation are manifestations of a rapid structural change of the cortical gel layer of the protoplasm, plasmalemma-ectoplasm complex. The importance of the rapid movement and rearrangement of water molecules in the cortical gel layer in association with excitation processes is emphasized.

Repetitive mechanical responses of the amphibian skin to adrenergic stimulation .

Mechanical responses of the amphibian nerve-skin preparation to adrenergic stimulation were investigated by recording pressure changes at the skin surface with a piezoelectric sensor. When a dilute epinephrine (or norepinephrine) solution was applied to the inner skin surface, repetitive mechanical responses, representing quick swelling of the skin repeating at more or less regular intervals of about 1 min, were frequently observed. In about 10% of the preparations, the skin was found to undergo repetitive quick shrinkage (instead of swelling) under practically indistinguishable experimental conditions. Rapid volume changes occurring in the cytoplasmic gel of the gland cells are considered to be at the base of these repetitive mechanical responses.

Rapid volume expansion in the Torpedo electric organ associated with its postsynaptic potential.

By electrically stimulating Torpedo electric organs enclosed in a sealed plastic chamber, it was demonstrated that the production of a postsynaptic potential is accompanied by a rapid increase in the volume of the organ. This increase started almost simultaneously with the onset of the postsynaptic potential and reached its maximum at about the end of the potential. The volume returned slowly to its resting level. Repetitive stimulation produced a summation of volume changes. The maximum change evoked by a single electric shock was roughly 10(-6) times the electrocyte volume. The thermal expansion of the organ associated with the production of a postsynaptic potential accounts for roughly 25% or less of this volume increase. The possibility is pointed out that a cation-exchange process involving Ca2+ on anionic sites in the electrocyte protoplasm may be at the base of the volume changes.

Rapid changes in light-scattering in the prism of Torpedo electric organ slice associated with the production of postsynaptic potentials.

Using a bifurcated lightguide placed on the surface of a prism of the electric organ slice of Torpedo, the generation of postsynaptic potentials by direct stimulation was found to be associated with readily detectable, disphasic optical changes. The changes consisted of a decrease in light-scattering during the rising phase of the electric potential, followed by a prolonged increase in scattering. The effects of varying the calcium-ion concentration in the medium strongly support the view the replacement of transmitter molecules for bound Ca-ions in the receptor proteins is at the base of these changes. The effects of synaptically active reagents on these optical signals were demonstrated.

Mechanical and thermal changes in the Torpedo electric organ associated with its postsynaptic potentials.

Rapid mechanical and thermal changes in slices of the Torpedo electric organ evoked by electric stimulation were investigated. The organ was found to swell simultaneously with the postsynaptic potential. This swelling was followed by prolonged shrinkage of the organ. These findings may be explained from the proven facts that the acetylcholine(ACh)-receptor proteins have a large binding capacity for Ca-ions and that addition of ACh to the medium can cause release of Ca-ions from the receptor proteins.

Temperature changes associated with excitation of the electric organ in the African electric catfish.

By using heat-sensors made from synthetic pyroelectric film, it was shown that excitation of the Malapterurus electric organ brings about first a rapid rise, then a slow fall and finally a very slow rise in the temperature of the organ. Difficulties encountered in determining these rapid temperature changes by the method of employing thermistor probes are pointed out.

Discontinuous volume transitions induced by calcium-sodium ion exchange in anionic gels and their neurobiological implications.

Discontinuous volume transitions in poly (acrylic acid) gels were studied by determining equilibrium swelling curves of small gel beads immersed in salt solutions containing varying concentrations of Ca2+ and Na+. Reversible contractions of gel membranes associated with Ca(2+)-Na+ exchange were examined using both isometric and isotonic recording devices. A discontinuous change in electric conductance was demonstrated in association with Ca(2+)-Na+ exchange in gel membranes. These experimental findings provide a sound physicochemical basis for elucidating the mechanism of nerve excitation.

Rapid heat production associated with excitation of electric organs of the electric eel.

By constructing a new type of thermal detector, rapid production of heat was demonstrated during the rising phase of the action potential of the eel electric organs evoked by direct electric stimulation. Following the indirect (synapse-mediated) action potential of the organs, the existence of very large and variable production of heat of unknown origin was revealed.

The origin of rapid changes in birefringence, light scattering and dye absorbance associated with excitation of nerve fibers.

By comparing the time-courses of rapid optical changes in the garfish olfactory nerve evoked by electric stimulation with those of mechanical changes observed at the site of optical recording, the origin of optical changes in the nerve has been investigated. Based on the finding that the time-course of the birefringence change accurately coincides with that of swelling of the nerve, optical changes are interpreted as being brought about by invasion of water into the superficial layer of the nerve fibers. A close relationship has also been demonstrated between nerve swelling and changes in light scattering and in dye absorbance.

Rapid structural changes in nerve fibers evoked by electric current pulses.

Using the garfish olfactory nerve, the time-courses of lateral expansion (swelling) and birefringence changes in nerve fibers have been examined at the site of application of electric current pulses. The effects of various chemical agents on these non-electrical signs of rapid structural changes in the nerve fibers have been examined. These studies have shown that a pulse of outwardly-directed current through the superficial layer evokes a fast increase, followed by a slow, gradual increase, in the water-content of the nerve fibers. The full significance of these findings in studies of the process of nerve excitation is discussed.

Discontinuous volume transitions in ionic gels and their possible involvement in the nerve excitation process.

Discontinuous volume changes in polymer gels carrying negatively ionized groups were studied by varying the molarities of univalent and bivalent cations in the bathing solution. These studies offer a sound basis for elucidating the origin of rapid swelling and heat production in nerve fibers associated with the process of excitation.

Heat production associated with a propagated impulse in bullfrog myelinated nerve fibers.

By using heat-sensors constructed with thin film of polyvinylidene fluoride, it was found possible to detect the heat generated by myelinated fibers in the bullfrog sciatic nerve in association with a propagated impulse. The quantity of heat generated (about 0.4 microcal/g at 4.5 degrees C) is roughly two orders of magnitude smaller than that observed in nerves containing only non-myelinated nerve fibers. The smallness of the heat observed is attributed to the localization of the heat sources at the nodes of Ranvier. The major portion of the heat generated is re-absorbed by the nerve.

Nitrogen and energy utilization in germ-free and conventional chicks at early stages of growth.

Influence of aging on N and ME utilization in germ-free and conventional (CV) chicks was investigated at Stage 1 (2 to 6 days old) and Stage 2 (10 to 14 days old). Chicks were fed diets for ad libitum consumption with protein concentrations of 116, 220, or 445 g/kg diet. The N utilization was not affected by the gut microflora at either stages, but it significantly decreased for chicks on the high-protein diet compared with those on the low-protein and adequate-protein diets. The ME values of diets were not influenced by treatments, except for the high-protein diet at Stage 1, in which ME was significantly reduced.

Demonstration of heat production associated with spreading depression in the amphibian retina.

Using thin film of synthetic pyroelectric material, polyvinylidene fluoride, sensitive heat-sensors were constructed for the purpose of detecting heat production associated with the phenomenon of spreading depression in isolated amphibian retinae. Measurements with these sensors revealed the existence of large heat production, which closely follows the electrical sign of spreading depression. Studies of the effects of chemical agents known to affect spreading depression have demonstrated the usefulness of heat measurements.

Thermal and mechanical responses of the amphibian skin to nerve stimulation.

The isolated calf skin of the toad and bullfrog was found to generate a burst of heat in response to stimulation of the cutaneous nerve. The electric responses of the skin evoked by nerve stimulation invariably lagged behind the heat burst. The generation of a heat burst was followed by a slow mechanical response of the skin. Suppression of these responses by adrenergic blocking agents suggests that heat is produced at the junction between the gland cells and the sympathetic nerve fibers. Large mechanical changes in the skin were observed when noradrenaline was applied to the inner surface of the skin.

Volume expansion of nonmyelinated nerve fibers during impulse conduction.

Nonmyelinated nerve fibers undergo rapid volume expansion while carrying an impulse. This volume expansion is incurred as a consequence of a lateral expansion of the excited portion of the fibers, where the superficial layer is transformed into a low-density structure.

Mechanical changes associated with synaptic transmission in the mammalian superior cervical ganglion.

Mechanical changes in the superior cervical ganglion (SCG) of the guinea pig, evoked by stimulating the pre- or postganglionic nerve, were determined by using a mechano-electric transducer. The mechanical change evoked by either one of these nerves was monophasic, an initial phase of rapid swelling (about 1 dyn/cm2) being followed by a slow relaxation. The mechanism of these mechanical changes was examined by treating the preparations with agents which are known to suppress or facilitate synaptic transmission in the SCG. Mechanical changes elicited by preganglionic nerve stimulation were suppressed by agents that reversibly block ganglionic transmission (Ca2(+)-free high Mg2+, d-tubocurarine, hexamethonium, TEA, TTX). On the other hand, agents which are known to facilitate synaptic transmission (high Ca2+) increased both the amplitude and the duration of swelling. In a Ba2(+)-containing medium, a large, long-lasting shrinkage of the ganglion was observed. It is suggested that the mechanical changes recorded in this study are associated with postsynaptic electro-genesis.