Main description:

In the past 5 years there has been an enormous increase of evidence that the ion channels activated by mechanical force are common to a wide variety of cell types. Mechanosensitive (MS) ion channels form a small proportion of the total channel population. They are now found in more than 30 cell types from E. coli, yeast, to plant, invertebrate, and vertebrate cells, where they occur in virtually all types of cells from bone to smooth muscle, as well as neurons. The majority of MS channels are permeable to monovalent cations and are slightly selective for K+ over Na +. How 2 ever, there are several reports of anion-selective MS channels, MS Ca + channels, and MS channels with large conductances that do not dis criminate markedly between cations and anions. Recently B. Hille has postulated possible evolutionary relationships between several types of ion channels, with mechanosensitive channels predating even the eukaryotes. Two voltage-gated channel types originate with the stem eukaryotes, as deduced from the presence of voltage-gated K+ 2 and Ca + channels in protozoa, algae, or higher plants. Agonist-gated chan nels as well as voltage-gated Na + channels appear with the earliest metazoan animals, as deduced from the presence of Na + spikes and fast chemical synapses in cnidaria (coelenterates), ctenophores, and all higher animals.

Contents:

I Mechanosensitive Ion Channels.- 1 Mechanosensitive Ion Channels in Bacteria.- 1 Introduction.- 2 Mechanosensitive Ion Channels in Escherichia coli.- 3 Mechanosensitive Ion Channels in Other Bacteria.- 4 Perspective.- References.- 2 Mechanosensitive Ion Channels in Yeast. Mechanisms of Activation and Adaptation.- 1 Introduction.- 2 Mechanisms of Activation of Mechanosensitive Channels.- 3 Mechanisms of Adaptation of Mechanosensitive Channels.- 4 Physiological Function of Mechanosensitive Channels in Walled Organisms.- 5 Summary.- References.- 3 Mechanosensory Transduction in Ciliates (Protozoa).- 1 Mechanoreception and Cellular Behaviour.- 2 Physiological Responses to Mechanical Stimulation.- 3 Properties of Mechanosensitive Ion Channels.- 4 Electrical Excitation: Voltage-Dependent Ion Channels.- 5 The Significance of Ion Channel Localization.- 6 Concluding Remarks.- References.- 4 Towards Molecular Mechanism of Activation in Mechanosensitive Ion Channels.- 1 Introduction.- 2 Difficulties in Studying Mechanosensitive Ion Channels.- 3 Analysis of SA Channel Gating Kinetics.- 4 Thermodynamic Models for SA Channel Gating.- 5 Measurement of Membrane Tension.- 6 Conclusions and Future Perspectives.- References.- II Mechanoreceptor Organs.- 5 Gravity Sensing by Higher Plants.- 1 Introduction.- 2 Mechanically Activated Channels in a Model Plant System.- 3 Possible Ways to Open Gravitransductive Channels.- 4 Distribution of Mechanical Stress and Channel Activation.- 5 Activation of Auxin Porters.- 6 Load-Bearing at the Cell Wall Level.- 7 Is Gravitropic Stress-Sensing an Expression of a More General Growth-Regulating System?.- 8 Hydrostatic Pressure.- 9 Statoliths.- References.- 6 Mechanoreceptors in Respiratory Systems.- 1 Introduction.- 2 Upper Airway Receptors.- 3 Mechanoreceptors of the Respiratory Muscles and Joints.- 4 Lower Respiratory Tract and Lungs.- 5 Conclusions.- References.- 7 Cardiovascular Mechanoreceptors.- 1 Introduction.- 2 Location and General Function of Arterial Baroreceptors.- 3 Mechanisms of Activation of Baroreceptors.- 4 Influences of Pulsatile Pressure and Flow on Baroreceptor Discharge.- 5 Adaptation and Resetting of Baroreceptors.- 6 Summary.- References.- 8 Comparative Physiology of Cutaneous Mechanoreceptors.- 1 History.- 2 Types of Receptors.- 3 Special Features in Different Species.- 4 Mechanical Properties and Method of Stimulation.- 5 Sympathetic Control of Mechanoreceptors.- References.- 9 Invertebrate Auditory Receptors.- 1 Introduction.- 2 Basic Structure and Physiology of Auditory Receptors.- 3 Airborne Sound Receptors.- 4 Vibration Receptors.- 5 Selected Reading.- References.- 10 Mechanoelectrical Transduction in Vertebrate Hair Cells.- 1 Introduction.- 2 Electrophysiology of Hair Cells.- 3 Mechanoelectrical Transduction.- 4 Muscarinic Cholinergic Receptor Mechanism Might Mediate the Efferent Innervation to the Hair Cell.- 5 Length Change of the Guinea Pig Outer Hair Cell.- References.- 11 Muscle Mechanoreceptors in Invertebrates.- 1 Introduction.- 2 Occurrence, Location and Gross Morphology of Muscle Receptors.- 3 Fine Structure and Organization of Muscle Receptor Organs.- 4 Functional Properties of Muscle Receptor Organs.- 5 Summary.- References.- 12 Muscle Mechanoreceptors in Nonmammalian Vertebrates.- 1 Introduction.- 2 Spindle Preparation and Parameters of Sensory Axon Terminal.- 3 Distinction Between Spindle Potential and the Actual Receptor Potential.- 4 The Site of Origin and Supposed Role of the Spindle Potential.- 5 A Model of Primary Processes in Sensory Encoding.- 6 Transduction and Mechanosensitive Channel: Ultrastructural Aspects of Frog Muscle Spindle.- 7 Second Messengers in Transduction and Encoding Processes.- References.- 13 Muscle Receptors in Mammals.- 1 Introduction.- 2 Types of Intrafusal Muscle Fibre.- 3 Fusimotor Innervation - The Basic Pattern.- 4 Effects of Contraction of Intrafusal Fibres on Spindle Sensory Endings.- 5 Nonselectivity and the Subdivision of Static ?-Axons.- 6 Summary and Conclusions.- References.