Mechanism and Function of the Catch State in Molluscan Smooth Muscle: A Historical Perspective

Molluscan smooth muscles exhibit the catch state, in which both tension and resistance to stretch are maintained with very low rates of energy consumption. The catch state is studied mainly on the anterior byssus retractor muscle (ABRM) of a bivalve molluscan animal, Mytilus, which can easily be split into small bundles consisting of parallel fibers. The ABRM contracts actively with an increase in the intracellular free Ca ion concentration, [Ca2+]i, as with all other types of muscle. Meanwhile, the catch state is established after the reduction of [Ca2+]i to the resting level. Despite extensive studies, the mechanism underlying the catch state is not yet fully understood. This article briefly deals with (1) anatomical and ultrastructural aspects of the ABRM, (2) mechanical studies on the transition from the active to the catch state in the isotonic condition, (3) electron microscopic and histochemical studies on the intracellular translocation of Ca ions during the transition from the active to the catch state, and (4) biochemical studies on the catch state, with special reference to a high molecular mass protein, twitchin, which is known to occur in molluscan catch muscles.

Head repositioning accuracy is influenced by experimental neck pain in those most accurate but not when adding a cognitive task

Background and aimsNeck pain can impair perception of cervical movement, but how this is affected by attention is unknown. In this study, the effects of experimental neck pain on head repositioning accuracy during standardized head movements were investigated.MethodsExperimental neck pain was induced by injecting hypertonic saline into the right splenius capitis muscle in 28 healthy participants (12 women). Isotonic saline was used as control. Participants were blindfolded while performing standardized head movements from neutral (start) to either right-rotation, left-rotation, flexion or extension, then back to neutral (end). Movements were triplicated for each direction, separated by 5-s, and performed with or without a cognitive task at baseline, immediately after the injection, and 5-min after pain disappeared. Repositioning accuracy was assessed by 3-dimensional recordings of head movement and defined as the difference between start and end position. Participants were grouped into most/least accurate based on a median split of head repositioning accuracy for each movement direction at baseline without the cognitive task.ResultsThe most accurate group got less accurate following hypertonic injection during right-rotation without a cognitive task, compared with the least accurate group and the isotonic condition (p < 0.01). No group difference was found when testing head repositioning accuracy while the participants where distracted by the cognitive task.ConclusionsExperimental neck pain alters head repositioning accuracy in healthy participants, but only in those who are most accurate at baseline. Interestingly, this impairment was no longer present when a cognitive task was added to the head repositioning accuracy test.ImplicationsThe results adds to our understanding of what factor may influence the head repositioning accuracy test when used in clinical practice and thereby how the results should be interpreted.

Regulation of V2R transcription by hypertonicity and V1aR-V2R signal interaction

Arginine vasopressin (AVP) and hypertonicity in the renal medulla play a major role in the urine concentration mechanism. Previously, we showed that rat vasopressin V2 receptor (rV2R) promoter activity was increased by vasopressin V2R stimulation and decreased by vasopressin V1a receptor (V1aR) stimulation in a LLC-PK1 cell line stably expressing rat V1aR (LLC-PK1/rV1aR). In the present study, we investigated the effects of hypertonicity on the rV2R promoter activity and on the suppression of rV2R promoter activity by V1aR stimulation in LLC-PK1/rV1aR cells. rV2R promoter activity was increased in NaCl- or mannitol-induced hypertonicity. The hypertonicity-responsive site in the rV2R promoter region was limited to 10 bp, including the Sp1 motif. The increase of V2R promoter activity by hypertonicity was significantly inhibited by a JNK inhibitor (SP600125) and PKA inhibitor (H89). In contrast, rV2R promoter activity was remarkably suppressed by V1aR stimulation in the hypertonic condition rather than in the isotonic condition. The AVP-stimulated intracellular Ca2+ concentration was increased in the hypertonic condition, suggesting the functional activation of V1aR by hypertonicity. In conclusion, 1) V2R promoter activity is increased by hypertonicity via the JNK and PKA pathways, 2) suppression of V2R expression by the V1aR-Ca2+ pathway is enhanced by hypertonicity, and 3) hypertonicity enhances the V1aR-Ca2+ pathway. The counteractivity of V2R and V1aR could be required to maintain minimum urine volume in the dehydrated state.

Quantitative Examination of a Perfusion Microscope for the Study of Osmotic Response of Cells

The perfusion microscope was developed for the study of the osmotic response of cells. In this microscope, the cells are immobilized in a transparent chamber mounted on the stage and exposed to a variety of milieus by perfusing the chamber with solutions of different concentrations. The concentration of the supplied solution is controlled using two variable-speed syringe pumps, which supply an isotonic solution and a hypertonic solution. Before using this system to characterize the osmotic response of cells, the change in the concentration of NaCl solution flowing through the chamber is examined quantitatively using a laser interferometer and an image processing technique. The NaCl concentration is increased from an isotonic condition to a hypertonic condition abruptly or gradually at a given constant rate, and decreased from a hypertonic condition to an isotonic condition. It is confirmed that the concentration is nearly uniform in the cross direction at the middle of the chamber, and the change in the NaCl concentration is reproducible. The average rate of increase or decrease in the measured concentration agrees fairly well with the given rate when the concentration is changed gradually at a constant rate. The rate of the abrupt change is also determined to be the highest limit achieved by the present method. As the first application of using the perfusion microscope for biological studies, the volume change of cells after exposure to a hypertonic solution is measured. Then, the hydraulic conductivity of the cell membrane is determined from the comparison of the volume change between the experiment and the theoretical estimation for the measured change in the NaCl concentration of the perfused solution.

Vibration-Induced Postural Posteffects

Wierzbicka, M. M., J. C. Gilhodes, and J. P. Roll. Vibration-induced postural posteffects. J. Neurophysiol. 79: 143–150, 1998. It generally is known that vibration of various muscles in free-standing subjects evokes a spatially oriented postural response. Furthermore, it recently has been shown that when a vibratory stimulus is terminated, a powerful involuntary contraction of the previously vibrated muscle often occurs that, under the isotonic condition, is accompanied by movement of a limb. The aim of this study was to explore effects of a low-amplitude mechanical vibration, applied in a seated position, on the standing posture. The 30-s vibration was applied bilaterally at the ankle level to anterior or posterior tendons and at the cervical level in front or back of the neck, at one site only at a time. Center of pressure trajectories were monitored during quiet stance for ≤19 min after the offset of vibration, and these measurements were compared with a previbration control trial. The results clearly indicate that vibration produced in all subjects strong, long-lasting dynamical modification of posture mainly in the anterior-posterior direction. Spatial orientation of the induced postvibratory shift in posture was dependent on the vibration side. We conclude that sustained Ia sensory inflow, evoked by vibration, has a powerful after-effect on the motor system at the postural level.

Hypotonicity increases basolateral taurine permeability in rabbit proximal convoluted tubule

The permeabilities of the basolateral membrane of rabbit proximal convoluted tubule (PCT) to taurine (PTau) and glucose (PGlc) were estimated under control and hypotonic conditions using the initial rate of increase in cellular volume (CV) induced on isotonic replacement of 40 mM mannitol by one or the other of these substrates. Under control conditions, addition of taurine led to an increase in CV at an initial rate of 7.1 +/- 1.7%/min, leading to a cell swelling of 30.2 +/- 4.8% after 5 min (n = 6). Addition of glucose led to an increase in CV at an initial rate of 30.0 +/- 3.8%/min, leading to a cell swelling of 25.7 +/- 3.1% after 5 min (n = 7). After a period of recovery of 5 min in the absence of taurine or glucose, a 40 mosmol/kg hypotonic shock induced a cell swelling of 14.2 +/- 1.3 and 16.1 +/- 5.2%, respectively, followed by an almost complete volume regulatory decrease after 5 min. At that time, addition of taurine under continuous hypotonicity induced an increase in CV at an initial rate 2.57 +/- 0.17 times larger than that observed under the isotonic condition (P < 0.005), while addition of glucose induced an initial increase in CV identical to that observed under the isotonic condition. The increases in CV observed on addition of taurine were completely abolished in the absence of sodium under both isotonic and hypotonic conditions. The permeability to K+ was also estimated, in the absence of sodium, using the initial rate of increase in CV induced on isotonic replacement of 40 mM N-methyl-D-glucamine by K+.(ABSTRACT TRUNCATED AT 250 WORDS)