Multiple Facets and Roles of Na+-K+-Cl- Cotransport: Mechanisms and Therapeutic Implications.

The Na+-K+-Cl- cotransporters play key physiological and pathophysiological roles by regulating the membrane potential of many cell types and the movement of fluid across a variety of epithelial or endothelial structures. As such, they should soon become invaluable targets for the treatment of various disorders including pain, epilepsy, brain edema, and hypertension. This review highlights the nature of these roles, the mechanisms at play, and the unresolved issues in the field.

K+-Cl- cotransporter 1 (KCC1): a housekeeping membrane protein that plays key supplemental roles in hematopoietic and cancer cells.

During the 1970s, a Na+-independent, ouabain-insensitive, N-ethylmaleimide-stimulated K+-Cl- cotransport mechanism was identified in red blood cells for the first time and in a variety of cell types afterward. During and just after the mid-1990s, three closely related isoforms were shown to account for this mechanism. They were termed K+-Cl- cotransporter 1 (KCC1), KCC3, and KCC4 according to the nomenclature of Gillen et al. (1996) who had been the first research group to uncover the molecular identity of a KCC, that is, of KCC1 in rabbit kidney. Since then, KCC1 has been found to be the most widely distributed KCC isoform and considered to act as a housekeeping membrane protein. It has perhaps received less attention than the other isoforms for this reason, but as will be discussed in the following review, there is probably more to KCC1 than meets the eye. In particular, the so-called housekeeping gene also appears to play crucial and specific roles in normal as well as pathological hematopoietic and in cancer cells.

Physiological roles and molecular mechanisms of K+ -Cl- cotransport in the mammalian kidney and cardiovascular system: where are we?

In the early 80s, renal microperfusion studies led to the identification of a basolateral K+ -Cl- cotransport mechanism in the proximal tubule, thick ascending limb of Henle and collecting duct. More than ten years later, this mechanism was found to be accounted for by three different K+ -Cl- cotransporters (KCC1, KCC3 and KCC4) that are differentially distributed along the renal epithelium. Two of these isoforms (KCC1 and KCC3) were also found to be expressed in arterial walls, the myocardium and a variety of neurons. Subsequently, valuable insights have been gained into the molecular and physiological properties of the KCCs in both the mammalian kidney and cardiovascular system. There is now robust evidence indicating that KCC4 sustains distal renal acidification and that KCC3 regulates myogenic tone in resistance vessels. However, progress in understanding the functional significance of these transporters has been slow, probably because each of the KCC isoforms is not identically distributed among species and some of them share common subcellular localizations with other KCC isoforms or sizeable conductive Cl- pathways. In addition, the mechanisms underlying the process of K+ -Cl- cotransport are still ill defined. The present review focuses on the knowledge gained regarding the roles and properties of KCCs in renal and cardiovascular tissues.

Modulation of corticospinal excitability during imagined knee movements.

In this study, we investigated corticospinal excitability during mental simulation of a leg extension movement with the technique of transcranial magnetic stimulation. Motor evoked potentials were recorded in both knee extensors (quadriceps) and flexors (biceps femoris) in 19 trained participants (healthy volunteers). The amplitude and latency of motor evoked potentials were compared in three conditions: (1) at rest, (2) during motor imagery, and (3) at rest, immediately after motor imagery. The results showed a significant effect (p < 0.001) of conditions on motor evoked potentials amplitude in the quadriceps but not in the biceps femoris. During motor imagery, the size of motor evoked potentials in the quadriceps increased significantly (p < 0.001) compared with rest and post-imagery conditions. Changes in motor evoked potentials latency across conditions were not significant, however. These results are consistent with previous studies in the upper limb and suggest that corticospinal excitability can be enhanced during motor imagery to facilitate responses in specific lower limb muscles.

Recovery time of independent poststroke life habits: Part II.

The purposes of this study were to determine the time of the recovery of poststroke life habits and to identify prognostic indicators associated with recovery time among stroke patients in a rehabilitation program. A sample of 421 stroke patients who were admitted to a rehabilitation center was recruited from medical records available from January 1987 to December 1992. The relation between the achievement of independent life habits including bed mobility, transfers and ambulation, bathing activities, dressing activities, eating activities, home activities, sphincter control, and sleep with the potential covariates associated with recovery time was assessed through the analysis of survival data using the Cox maximum-likelihood proportional hazard models. The poststroke life habits obtained generated mean recovery times ranging from 5.51 to 57.60 days from admission to rehabilitation. The survival analysis revealed that the recovery time of the selected poststroke abilities was significantly influenced (p <.05) by one or several indicators; these included physical ability and neuropsychological and life habit characteristics. With this precious information, stroke rehabilitation specialists may be able to reduce the length of time required to recover independent poststroke life habits by treating the specific neuropsychological, physical, and life habit characteristics identified in this study. A faster poststroke recovery would reduce the socioeconomic impact generated by stroke disability and would also ensure a better quality of life to the stroke survivor.

Recovery time of independent poststroke abilities: Part I.

The purposes of this study were to determine the time of the recovery of poststroke abilities and to identify prognostic indicators associated with recovery time among stroke patients undergoing a rehabilitation program. A sample of 421 stroke participants admitted to a rehabilitation center was recruited from medical records that were available from January 1987 to December 1992. The mean age was 61.8 years (range, 17-89 years). The relationship between the achievement of independent poststroke abilities and the potential covariates associated with recovery time was assessed through the analysis of survival data. Cox maximum-likelihood proportional hazard models were used for the analysis. Independent poststroke abilities included behavior, cognitive, perceptual, communication, visual, and motor status. The time from rehabilitation admission to complete independence was introduced to the model in relation to the covariates. The mean time of recovery of poststroke abilities ranged from 18.70 to 32.40 days from the rehabilitation admission. The survival analysis revealed that the time of recovery of the selected poststroke abilities was significantly influenced (p <.05) by one or several factors, among these were neuropsychological, physical, and life habits. With this precious information, stroke rehabilitation specialists may be able to reduce the length of time required to recover independent poststroke abilities by treating the specific neuropsychological, physical, and life habit characteristics identified in this study. A faster poststroke recovery will reduce the socioeconomic impact generated by stroke disability and will ensure a better quality of life to the stroke survivor.

Action of 5-hydroxytryptamine, substance P, thyrotropin releasing hormone and clonidine on spinal neuron excitability.

We investigated the influence of four substances on the excitability of lumbar motoneurons. These substances, three of which coexist in the same bulbospinal descending pathways that end, for the most part, around motoneurons (MNS), are: 5-hydroxytryptamine (5-HT), substance P (SP) and thyrotropin-releasing hormone (TRH). We also studied the effects of clonidine, an alpha 2 noradrenergic (NA) agonist. This study was carried out in rats spinalized at T5 and treated three weeks earlier with 5-7 dihydroxytryptamine (5-7 DHT). Under these conditions, the following responses were observed: 5-HTP (5-HT precursor) intraperitoneally (I.P.), 5-HT intrathecally (I.T.), TRH (I.P. or I.T.) and substance P (I.T.) all elicited strong excitation of MNS as measured by integrated EMG of the hindlimb muscles; substance P reduced by almost half the response to 5-HTP given one hour and 24 hours later; TRH given acutely did not modify the response to 5-HTP, but given chronically for 21 days markedly increased the response to this substance. Clonidine by itself decreased the excitability of MNS and antagonized the excitatory effects of 5-HTP and TRH. In two separate pilot trials, cyproheptadine, a 5-HTP antagonist, decreased the manifestations of spasticity in a patient with a partial spinal lesion. It would appear that clonidine may have potential use in the management of spasticity.

Chronic administration of thyrotropin-releasing hormone enhances the sensitivity of lumbar motoneurons to 5-hydroxytryptophan in the rat.

The influence of a single intraperitoneal injection or of a three-week intrathecal infusion of thyrotropin-releasing hormone (TRH) was studied on the excitatory effect of DL-5-hydroxytryptophan (5-HTP) (20-100 mg/kg IP) on lumbar motoneurons. This effect was studied by recording the integrated spontaneous electromyographic (EMG) activity of the hindlimb muscles in spinalized rats previously denervated with an intrathecal injection of 5,7-dihydroxytryptamine. A single injection of TRH 10 mg/kg IP caused by itself a stimulation of the EMG activity but failed to modify the subsequent EMG response to 5-HTP one hour or one day later. However, a three-week infusion of TRH in the lumbar subarachnoid space caused a 300% increase in the response to 5-HTP, while denervation alone caused only an increase of 160%. A similar potentiation of the effect of 5-HTP was seen on the Wet Dog Shake phenomenon which we believe is elicited in the forebrain. Therefore, chronic but not acute stimulation of lumbar motoneurons or forebrain structures by TRH appears to facilitate the serotonergic excitating response. This effect of TRH does not appear to necessitate the actual coexistence of 5-HT and TRH in the same nerve fibers.

Action of 5-hydroxytryptamine, substance P, thyrotropin-releasing hormone and clonidine on motoneurone excitability.

We have investigated the influence on the excitability of lumbar motoneurons of 5-hydroxytryptamine (5-HT), substance P and thyrotropin releasing hormone (TRH), three substances which coexist in the same bulbospinal descending pathway and end in large part around motoneurons. We have also studied the effect of clonidine, an alpha 2 noradrenergic agonist. This was done in spinalized rats (T5) treated three weeks before with 5-7-dihydroxytryptamine. Under those circumstances 5-HTP (I.P.), 5-HT (intrathecally) TRH (I.P. or I.T.) and substance P (I.T.) all elicited a strong excitation of motoneurons as measured by integrated EMG of the hindlimb muscles. Substance P reduced by almost half the subsequent response to 5-HTP, 1 hour and 24 hours later. TRH given acutely did not modify the response to 5-HTP but given chronically for twenty one days by means of Alzet minipump, markedly increased the response to 5-HTP. Clonidine by itself decreased the excitability of motoneurons and antagonized the excitatory effect of 5-HTP and TRH. In a pilot trial, cyproheptadine, a 5-HT antagonist was shown to decrease the manifestations of spasticity in patients with a partial spinal lesion. Clonidine also appears to be of potential use in the treatment of spasticity.

Substance P in the lumbar spinal cord of the rat affects the motor response to 5-HTP and TRH.

Substance P, administered intrathecally in the lumbar area of paraplegic rats activated the electromyogram (EMG) of the hindlimbs. This effect was markedly enhanced by previous denervation by 5,7-dihydroxytryptamine. Stimulation of the motoneurons by substance P was followed by a blunted response to the excitatory substances thyrotropin releasing hormone (TRH) and 5-hydroxytryptophan (5-HTP), given intraperitoneally 1 hr later. The decreased sensitivity to 5-HTP persisted for more than 24 hr. In several animals, however, a flaccid paralysis was observed after administration of substance P. In such cases the response to 5-HTP was enhanced 24 hr later. These observations suggest an interaction between 5-HT, TRH and substance P in the control of the excitability of motoneurons.

Effect of clonidine on motoneuron excitability in spinalized rats.

In a dose of 0.1 mg/kg clonidine, an alpha-2 receptor agonist, depressed the spontaneous EMG activity of the biceps and quadriceps femoris in chronically-spinalized rats. It also antagonized in a dose-dependent manner the stimulating effect of 5-hydroxytryptophan (5-HTP, 100 mg/kg). Doses of more than 0.1 mg/kg were less potent in antagonizing the effect of 5-HTP. Clonidine reduced the tonic activity of the hindlimb muscles but allowed walking movements. The depressant effect of clonidine in animals pretreated with 5-HTP was prevented by yohimbine (1.25 mg/kg), while the depressant action of the serotonin antagonist, cyproheptadine was not. In chronically-spinalized rats, clonidine (0.1 mg/kg) increased the threshold of electrically-induced flexor and extensor reflexes and decreased their amplitude. No significant modification of reflexes was seen with this dose 24 hr after spinalization. Thus, clonidine in doses of 0.1 mg/kg or less reduced directly or indirectly the excitability of motoneurons. Clonidine may prove to be a useful therapeutic adjunct in the treatment of spasticity.

Denervation supersensitivity to 5-hydroxytryptamine in the rat spinal cord is not due to the absence of 5-hydroxytryptamine.

D,L-5-Hydroxytryptophan and 5-HT agonists administered systemically, stimulate motoneuronal discharges as measured by the spontaneous EMG activity of the hindlimbs in paraplegic rats. Denervation supersensitivity is observed after surgical section of the spinal cord or after treatment with 5,7-dihydroxytryptamine (5,7-DHT). Such denervation supersensitivity, however, cannot be reproduced by equivalent depletion of 5-HT by synthesis inhibition or reversed by chronic intrathecal administration of 5-HT agonists. These results suggest that in the anterior horn of the spinal cord, the trigger of denervation supersensitivity to serotonin is not the absence of the neurotransmitter itself but the absence of the terminals or some other compound contained therein.

Modifications occurring in motor programs during learning of a complex task in man.

This study was designed to investigate the modifications due to learning occurring in the motor program controlling a maximum speed arm adduction/forearm flexion movement. Twenty male subjects participated in 8 training sessions involving a total of 800 repetitions of the bi-articular movement. Significant improvements in arm adduction and forearm flexion speed as well as in posterior deltoid and triceps brachii latencies attributable to learning were found. Pectoralis major and biceps brachii motor times and posterior deltoid and triceps brachii activity times were not affected by training. The results support the hypothesis that a pre-established motor program exists in the cerebellum for bi-articular movements and that subroutines within this program control the agonist-antagonist coupling within each joint.