Christer Swerup

The permeability of the transducer membrane of the crayfish stretch receptor to calcium and other divalent cations.

Abstract The generator potential of the crayfish stretch receptor is produced by the movement of cations through a channel which is opened by stretch. Physiologically, Na + is the main ion moving through the channel; the channel is sufficiently large to admit arginine. The permeability to divalent cations has been measured in solutions in which all of the Na + was replaced by these ions. Ca 2+ , Mg 2+ , Sr 2+ and Ba 2+ were able to move through the channel, as shown by the membrane potential responses to stretch in solutions in which these ions were predominant. Ca 2+ (13.5 min) was found to be necessary in the Ba 2+ and Mg 2+ solutions to maintain membrane integrity; however, in the Sr 2+ solution Ca 2+ was not required. The results of voltage clamp studies showed the resting conductance to decrease in the divalent cation solutions. The reversal potential for the stretch induced current, measured with voltage clamp, was shifted in the negative direction by Ca 2+ , Sr 2+ and Mg 2+ . Two estimates of the value of the ratio of the permeability of Ca 2+ to that of Na + , calculated using the Goldman-Hodgkin-Katz equation, and either the change in reversal potential or the stretch induced current were 1.4 and 0.3 respectively.

Studies on the role of calcium in adaptation of the crustacean stretch receptor. Effects of intracellular injection of calcium, EGTA and TEA

The effects of intracellular pressure injection of Ca2+, EGTA and TEA on the receptor potential of the crayfish stretch receptor were studied. Injection of Ca2+ caused both the transient phase and the static phase of the receptor response to diminish in amplitude, the decrease being greater for the static phase. This phase was almost abolished after a few minutes of injection. Injection of EGTA caused a decrease in the amplitude of the transient phase and an increase of the static phase. These changes progressed during the injection and finally the receptor potential became almost square. After injection of TEA the static phase increased and approached the height of the transient phase making the response almost square. The results provide evidence for the important role of intracellular Ca2+ for the adaptation of the receptor. It is suggested that the adaptive decline of the receptor potential is due to an outward potassium current which is controlled by the intracellular concentration of Ca2+.

Effects of intracellular TEA injection on early adaptation of crustacean stretch receptor

The effects of intracellular injection of TEA on the stretch-induced response of the slowly adapting stretch receptor of the crayfish have been examined to determine the contribution of an outward potassium current to the early adaptation of the neuron. Intracellular recording techniques including potential clamp measurements of membrane currents have been used. Injection of small amounts of TEA caused a pronounced depolarization of the neuron. In the early stage of depolarization there was a marked increase of the static phase of the response while the dynamic phase remained unchanged. When the resting membrane potential was kept constant by current injection both the dynamic phase and the static phase increased. However, the increase of the static phase was more pronounced than that of the dynamic phase and as a result the early phase of adaptation was almost abolished. Following TEA injection the reversal potential for both the dynamic phase and the static phase of the receptor current became somewhat more positive. TEA injection also reduced the outward current induced by a depolarizing potential step. The present results provide additional support for the hypothesis that the early phase of adaptation of the crustacean stretch receptor is attributed mainly to an outward potassium current.

Ionic dependence of early adaptation in the crustacean stretch receptor

In the present study we have examined the effects of changes in potassium and calcium concentration on the early adaptation of the slowly adapting stretch receptor of the crayfish using intracellular recordings including the potential clamp technique. This was because previous studies had suggested that the early adaptative decline of the receptor potential may be attributed mainly to ionic mechanisms involved in the transducer process. During prolonged exposure to K-free saline the cell depolarized; the early adaptive fall of the receptor potential was reduced and finally the response became almost rectangular. These effects developed more rapidly if the concentration of Ca was reduced in the K-free saline. It was shown by injection of current that the effects were not potential dependent. Removal of Ca reduced the amplitude of both the dynamic and static phase of the receptor potential. Isotonic Ca-saline suppressed the static phase of the receptor potential and prolonged exposure completely abolished the response. Potential clamp experiments demonstrated that in the Ca-free saline the passive membrane conductance increased; the static phase of the receptor current increased while the peak current decreased somewhat. In the K-free and Ca-free saline both phases of the receptor current increased. The present results support earlier findings that the major part of the early adaptive fall of the receptor potential is caused by an outward K+ current. Ca2+ modifies the adaptive fall and the static phase, most likely by activation of a Ca2+-dependent K+ current and/or by inactivation of the Na+ current.

Acute effects of intravenous injection of beta‐adrenoreceptor‐ and calcium‐channel antagonists and agonists in myasthenia gravis

The effect of intravenous injection of propranolol, verapamil, terbutaline, calcium, and edrophonium on neuromuscular transmission has been studied with repetitive nerve stimulation and clinical tests in 10 patients with myasthenia gravis (MG). The drugs were given intravenously in doses commonly used in clinical practice. Only minor clinical effects were noted except for edrophonium. The mean decrement of the deltoid muscle was not significantly changed after injection of propranolol (before, 31%; 15 min after injection, 27%) and verapamil (before, 29%; 15 min after injection, 26%). Terbutaline applied after propranolol and calcium applied after verapamil improved the decrement substantially. Edrophonium applied after propranolol or verapamil also greatly improved the decrement. We conclude that there is no rapid deterioration of neuromuscular transmission in patients with moderately severe MG after injections with therapeutic doses of propranolol and verapamil. However, we do not know if the most severely disabled MG patient could have reacted otherwise. We consider that, in cardiovascular emergencies, propranolol and verapamil may be used even in severe MG but with resuscitation equipment as well as specific antidotes available.

Mechanotransduction and the crayfish stretch receptor.

Mechanotransduction or mechanosensitivity is found in almost every cell in all organisms from bacteria to vertebrates. Mechanosensitivity covers a wide spectrum of functions from osmosensing, cell attachment, classical sensory mechanisms like tactile senses in the skin, detection of sound in hair cells of the hearing apparatus, proprioceptive functions like recording of muscle length and tension in the muscle spindle and tendon organ, respectively, and pressure detection in the circulation etc. Since most development regarding the molecular aspects of the mechanosensitive channel has been made in nonsensory systems it is important to focus on mechanosensitivity of sensory organs where the functional importance is undisputed. The stretch receptor organ of the crustaceans is a suitable preparation for such studies. The receptor organ is experimentally accessible to mechanical manipulation and electrophysiological recordings from the sensory neuron using intracellular microelectrode or patch clamp techniques. It is also relatively easy to inject substances into the neuron, which also makes the neuron accessible to measurements with fluorescent techniques. The aim of the present paper is to give an up to date summary of observations made on the transducer properties of the crayfish stretch receptor (Astacus astacus and Pacifastacus leniusculus) including some recent unpublished findings. Finally some aspects on future line of research will be presented.