Vascular endothelium is critically involved in the hypotensive and hypovolemic actions of atrial natriuretic peptide

Atrial natriuretic peptide (ANP), via its vasodilating and diuretic effects, has an important physiological role in the maintenance of arterial blood pressure and volume. Its guanylyl cyclase-A (GC-A) receptor is highly expressed in vascular endothelium, but the functional relevance of this is controversial. To dissect the endothelium-mediated actions of ANP in vivo, we inactivated the GC-A gene selectively in endothelial cells by homologous loxP/Tie2-Cre-mediated recombination. Notably, despite full preservation of the direct vasodilating effects of ANP, mice with endothelium-restricted deletion of the GC-A gene (EC GC-A KO) exhibited significant arterial hypertension and cardiac hypertrophy. Echocardiographic and Doppler flow evaluations together with the Evan's blue dilution technique showed that the total plasma volume of EC GC-A KO mice was increased by 11-13%, even under conditions of normal dietary salt intake. Infusion of ANP caused immediate increases in hematocrit in control but not in EC GC-A KO mice, which indicated that ablation of endothelial GC-A completely prevented the acute contraction of intravascular volume produced by ANP. Furthermore, intravenous ANP acutely enhanced the rate of clearance of radio-iodinated albumin from the circulatory system in control but not in EC GC-A KO mice. We conclude that GC-A-mediated increases in endothelial permeability are critically involved in the hypovolemic, hypotensive actions of ANP.

Hypervolemic hypertension in mice with systemic inactivation of the (floxed) guanylyl cyclase-A gene by alphaMHC-Cre-mediated recombination

To dissect the tissue-specific functions of atrial natriuretic peptide (ANP), we recently introduced loxP sites into the murine gene for its receptor, guanylyl cyclase-A (GC-A), by homologous recombination (tri-lox GC-A). For either smooth-muscle or cardiomyocyte-restricted deletion of GC-A, floxed GC-A mice were mated to transgenic mice expressing Cre-recombinase under the control of the smooth-muscle SM22 or the cardiac alphaMHC promoter. As shown in these studies, Cre-mediated recombination of the floxed GC-A gene fully inactivated GC-A function in a cell-restricted manner. In the present study we show that alphaMHC-Cre, but not SM22-Cre, with high frequency generates genomic recombinations of the floxed GC-A gene segments which were transmitted to the germline. Alleles with partial or complete deletions were readily recovered from the next generation, after segregation of the Cre-transgene. We took advantage of this strategy to generate a new mouse line with global, systemic deletion of GC-A. Doppler-echocardiographic and physiological studies in these mice demonstrate for the first time the tremendous impact of ANP/GC-A dysfunction on chronic blood volume homeostasis.

Influence of spinal cord injury on the morphology of bladder afferent and efferent neurons

Severe micturition dysfunction can occur following spinal cord injury (SCI) due to abnormal contractions of the urethral sphincter during bladder contractions (bladder/sphincter dyssynergia). This causes urinary retention, bladder overdistension, and increases the workload of the bladder leading to hypertrophy of the bladder muscle. Bladder hypertrophy induced by urethral outlet ligation in rats is accompanied by enlargement of both the afferent and efferent neurons innervating the bladder. The primary aim of this study was to test whether SCI-induced bladder hypertrophy produces a similar enlargement of bladder afferent neurons in the dorsal root ganglia (DRG) or efferent neurons in the major pelvic ganglia (MPG). Following SCI in female Wistar rats, there was a four-fold increase in bladder weight. The mean cross-sectional area of bladder DRG cell profiles increased approx. 50% after SCI; however, the mean area of MPG cell profiles did not change significantly. Urinary diversion (disconnecting the ureters from the bladder) prevented both the bladder hypertrophy and the DRG cell hypertrophy after SCI, suggesting that bladder hypertrophy drives DRG cell enlargement. On the other hand, since the size of MPG cells did not change significantly after SCI, bladder hypertrophy does not mandate MPG cell enlargement. However, preliminary results indicate that the mean cross-sectional area of MPG cells did increase (2-3 times) in SCI rats when the neural input to the MPG was eliminated by transecting the pelvic and hypogastric nerves; this suggests that the lack of change in size of MPG cells after SCI may be due to an inhibitory influence from the spinal cord.

Neural control of urethral outlet activity in vivo: role of nitric oxide

The present study investigated the role of nitric oxide (NO) in the reflex changes in urethral outlet activity during micturition. Isovolumetric bladder contractions, urethral pressure and external urethral sphincter electromyogram (EUS EMG) activity were recorded independently in urethane-anesthetized rats. During reflex bladder contractions, the urethra exhibited reflex responses characterized by an initial decrease in urethral pressure in conjunction with a rise in bladder pressure. This was followed by a period of high frequency oscillations (HFOs) associated with maximal urethral relaxation and burst type EUS EMG activity. Administration of N-nitro-L-arginine (L-NOARG) 10 mg./kg. intravenously, a nitric oxide synthase inhibitor, reversibly decreased the magnitude (62%, p < 0.05) and duration (40%, p < 0.05) of reflex urethral relaxation (N = 7). In 4 additional experiments, L-NOARG (10 to 15 mg./kg. intravenously) completely eliminated reflex urethral relaxation during micturition, and this effect was reversed in all animals by the administration of L-arginine (100 to 150 mg./kg. intravenously). Administration of N-nitro-D-arginine (D-NOARG) (10 to 30 mg./kg. intravenously) had no effect on reflex urethral relaxation. Neuromuscular blockade (vecuronium bromide 5 mg./kg. intravenously) reversibly decreased resting urethral pressure and eliminated the HFOs. The urethral smooth muscle relaxation that remained after neuromuscular blockade was eliminated following administration of L-NOARG (10 mg./kg. intravenously) in 2 of 3 animals. These results suggest that reflex urethral responses during micturition involve changes in both smooth and striated muscle activity, and that the predominant neurotransmitter mechanisms that mediate reflex urethral smooth muscle relaxation involve NO.

Effect of urinary diversion on the recovery of micturition reflexes after spinal cord injury in the rat

Patients with suprasacral spinal cord injury usually exhibit severe lower urinary tract dysfunction, which is generally attributed to loss of supraspinal input to the spinal micturition centers. However, some of the dysfunction may also arise secondary to bladder overdistension during the initial period of bladder areflexia. This study evaluated the consequences of bladder overdistension by performing urinary diversion in spinalized (T8-T10) rats. Bladder function was evaluated in urethane-anesthetized control and spinalized animals approximately 24 days after diversion. Chronically spinalized diverted and nondiverted rats exhibited similar micturition dysfunction: bladder/sphincter dyssynergia, incomplete voiding and ineffective (nonvoiding) bladder contractions. These data indicate that neither the condition of the bladder (such as chronic overdistension or bladder hypertrophy) nor afferent input from the bladder to the spinal cord dictates the development of reflex micturition and micturition dysfunction after spinal cord injury, suggesting that the dysfunction is intrinsic to spinal micturition reflex pathways.

Consequences of spinal cord injury during the neonatal period on micturition reflexes in the rat

This study examined the changes in micturition reflexes following spinal cord transection in neonatal rats to determine: (1) whether injury to the immature nervous system allows greater recovery of function than injury in adult animals and (2) whether the management of the lower urinary tract during the initial period following spinal injury influences the subsequent recovery of function. In one experiment, bladder-to-bladder reflexes in decerebrate neonatal rats (Day 15-Day 26) were tested 5-11 days after T8-T10 spinalization. While there was no difference in the amplitude and duration of reflex bladder contractions or bladder capacity between these pups and their nonspinalized controls, the spinalized pups exhibited incomplete voiding and an uncoordinated urethral sphincter (bladder/sphincter dyssynergia). It is concluded that the dyssynergia is inherent to the spinal micturition reflexes and is not due to an initial period of bladder areflexia and overdistension since in both the control and spinalized neonates micturition is initiated by a somatobladder reflex triggered when the mother licks the perineum. A second experiment tested whether neonatal spinal cord injury led to improved bladder function in adulthood. Postnatal Day 1 rat pups were spinalized at T8-T10 and returned to their mothers for the remainder of the neonatal period, and their bladder reflexes were tested 4-6 months later under urethane anesthesia. These rats showed the same lower urinary tract dysfunctions (bladder/sphincter dyssynergia, high residual volumes, decreased percentage voided volumes, and large-amplitude, long-duration bladder contractions) as adult rats that were spinalized as adults.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in Fluorogold and wheat germ agglutinin-horseradish peroxidase labelling of bladder afferent neurons

Rat urinary bladder afferent neurons were significantly smaller (34%) when labelled with Fluorogold (FG) than when labelled with wheat germ agglutinin-horseradish peroxidase (WGA-HRP). This study showed that this difference was due to an artifact of tissue processing (ethanol dehydration) and was not due to uptake and transport of the two tracers by two different subpopulations of bladder afferents.

Changes in bladder and external urethral sphincter function after spinal cord injury in the rat

Spinal cord injury (SCI) in humans results in inappropriate contractions of the external urethral sphincter muscle (EUS) during micturition (bladder-sphincter dyssynergia), leading to urinary retention. The major goal of this study was to determine whether SCI in rats has similar detrimental effects on micturition. After chronic SCI, urethan-anesthetized rats had a significantly (15-fold) increased bladder capacity and impaired voiding (31-fold increase in residual volume) compared with control rats. Bladder contractions in SCI rats were accompanied by abnormal tonic EUS electromyographic activity, whereas the EUS electromyograms of control rats exhibited a burst pattern (4-8 Hz) during voiding. Suppression of EUS activity with neuromuscular blockade did not improve the fraction of urine voided in SCI rats and reduced the fraction voided in control rats. Therefore, both tonic activity and complete quiescence of the rat's EUS appear to be detrimental to voiding, suggesting that the normal bursting EUS activity facilitates bladder emptying. In summary, rats and humans exhibit similar micturition dysfunctions after SCI (e.g., bladder-sphincter dyssynergia and impaired voiding).

Spinal pathways mediate coordinated bladder/urethral sphincter activity during reflex micturition in decerebrate and spinalized neonatal rats

Coordination between the urinary bladder and the external urethral sphincter is necessary for normal voiding. However, it is uncertain whether the spinal cord or brainstem generates this coordination. Bladder and urethral sphincter activity were examined during reflex voiding induced by perineal stimulation or bladder distension in decerebrate non-spinalized and spinalized 15 to 26-day-old neonatal rats. Perineal stimulation induced voiding and coordinated bladder/sphincter activity in both types of rats, indicating that spinal pathways can generate coordinated voiding behavior. The discoordination observed during voiding induced by bladder distension in spinalized pups may be due to the loss of descending pathways or to the emergence of detrimental spinal reflexes.

Modulation of the spinobulbospinal micturition reflex pathway in cats

Micturition, which is mediated by a spinobulbospinal reflex pathway, can be modulated by various spinal and supraspinal mechanisms. This study examined modulation of the micturition reflex in decerebrate unanesthetized cats. Electrical stimulation of the pontine micturition center (PMC) elicited two types of bladder responses: small-amplitude short-duration responses due to direct activation of the bulbospinal pathway (PS-direct contractions) and large-amplitude long-duration reflex responses induced by PS-direct contractions but maintained by afferent feedback (PS-reflex contractions). Rectal and vaginal-cervical stimulation inhibited the PS-direct contractions, indicating inhibition of the descending or efferent limb of the micturition pathway. Stimulation of the central end of a transected S2 ventral root elicited recurrent inhibition of PS-reflex contractions but not of PS-direct contractions, indicating that recurrent inhibition does not directly affect the descending pathway. Continuous electrical stimulation (20 Hz) of the PMC decreased (53 +/- 21%) bladder capacity, presumably by affecting transmission in the pons or ascending input to the pons. Thus the micturition reflex could be modulated at several sites: the pons, the ascending or descending pathways, or spinal interneuronal sites.

Components of the dynamic response of mammalian muscle spindles that originate in the sensory terminals

One component of the dynamic response of muscle spindles is characterized by a phase lead and frequency dependent sensitivity in response to sinusoidal stretches at frequencies around 1 Hz. Possible mechanisms producing this component, designated the "mid-frequency" dynamics, were investigated by testing the hypotheses that they arise from the mechanical behavior of the intrafusal muscle and alternatively from within the sensory terminals. Destruction of the myofibrillar structure of the intrafusal muscle fibers did not alter the mid-frequency dynamics, indicating that they do not arise from viscoelastic properties of the intrafusal muscle. An Arrhenius plot of the temperature dependence of the mid-frequency dynamics yielded an equivalent activation energy of 6.5 Kcal/M in the temperature range 23-42 degrees C and a 3-fold higher activation energy at lower temperatures. These observations are consistent with a dynamic process associated with a membrane-bound biochemical process. The addition of Ca++ and Ca(++)-activated-K+ (K(Ca] channel blockers (ZnCl2, Apamin and TEA) to the bathing solution altered the response dynamics by reducing the mid-frequency phase lead. The results suggest a negative feedback on the membrane potential generated by K+ efflux following a Ca++ influx that opens K(Ca) channels. A quantitative model fit to the experimental data yields a time constant of about 80 ms representing the limiting process associated with activation of the K(Ca) channels in this system. The results indicate that the mechanism underlying the mid-frequency dynamics includes at least two processes: one, not identified in this study, generates the phase lead and another, involving Ca++ and K(Ca) channels, provides a negative feedback that modifies the phase lead.

Properties of the descending limb of the spinobulbospinal micturition reflex pathway in the cat

The micturition reflex is thought to be mediated by a spinobulbospinal reflex pathway passing through the rostral pons. This study examined the properties of the descending limb of the reflex pathway by monitoring the responses of the lower urinary tract to stimulation of the pons in the decerebrate cat. Electrical stimulation (300 microseconds pulses at 50 Hz intratrain frequencies, 300-500 ms trains, 0.5-15 V) in the region of the locus coeruleus (P 0.5-3.1/L 2-4/H to -2.75) was used to activate the descending excitatory pathway to the sacral parasympathetic nucleus. Low intensity stimulation induced small amplitude, short duration (14 +/- 11 cm H2O, 10 +/- 3 s) bladder contractions in a partially full bladder, whereas higher intensity stimulation induced large amplitude, long duration (69 +/- 29 cm H2O, 70 +/- 44 s) contractions which were similar to distension-induced reflex micturition contractions. The evoked bladder contractions coincided with a reduction in external urethral sphincter (EUS) EMG activity. Following bilateral L7-S3 dorsal root transection, electrical stimulation of the pons still elicited the small amplitude bladder contractions, but the larger amplitude, long duration micturition contractions were abolished. During these small evoked bladder contractions, a suppression of EUS activity still occurred following deafferentation, indicating a pontine mediated bladder/EUS synergy. It is concluded that the pons can initiate bladder contractions and coordinated bladder-sphincter activity, but that afferent feedback (via the dorsal roots) is needed to maintain the large amplitude micturition contractions.

Pontine control of the urinary bladder and external urethral sphincter in the rat

Neurons in the rostral pontine tegmentum are known to have an important role in controlling micturition. The present experiments used urethane anesthetized rats to examine the effects of electrical stimulation at various sites in the pons on bladder and external urethral sphincter activity and on the volume threshold for inducing micturition. Stimulation with short trains of pulses (50 Hz, 1-3 s trains, 1-15 V) in the laterodorsal tegmental nucleus (LDT), the periaqueductal grey (PAG) or the lateral parabrachial nucleus (L-PBN) elicited contractions of a partially filled, quiescent bladder. However stimulation during a bladder contraction aborted the contraction indicating that these areas have inhibitory as well as excitatory effects. Continuous stimulation (50 Hz) in the PAG or L-PBN during a cystometrogram decreased bladder capacity (mean decrease 36%). Conversely, continuous stimulation in the pontine reticular formation (in or near the dorsal subcoeruleus nucleus and medial parabrachial nucleus) increased bladder capacity (mean increase 50%). Stimulation at pontine sites (LDT, PAG and L-PBN) which elicited bladder contractions also elicited an increase in external urethral sphincter activity. A similar increase in urethral sphincter activity occurred during reflex micturition induced by bladder distension. These data suggest that bladder capacity and the coordination of bladder and external urethral functions are controlled by various neuronal populations in the rostral pons of the rat.

Micturition reflexes in decerebrate and spinalized neonatal rats

Micturition in neonatal rats is mediated by a spinal reflex pathway activated by the mother licking the perineum (the perineal-to-bladder reflex, P-Bld). Micturition in adult rats is mediated by a spinobulbospinal reflex pathway activated by bladder distension (the bladder-to-bladder reflex, Bld-Bld). This study examines the postnatal development of the Bld-Bld reflex in decerebrate or spinalized unanesthetized and urethan-anesthetized rat pups 2-26 days of age. Urethan anesthesia depressed both the Bld-Bld and P-Bld micturition reflexes. Bld-Bld micturition reflexes (peak intravesical pressures of 13 +/- 6 cmH2O and durations of 35 +/- 11 s) were noted in 54% of 2-day-old decerebrate pups and in all of the 6- and 9-day-old decerebrate pups but in none of the 2- or 9-day-old spinalized pups. We conclude that a weak supraspinal Bld-Bld reflex is present during the early postnatal period; however, the P-Bld reflex is the primary mediator of micturition in the neonatal rat.

Electrical 'tuning' in muscle spindle receptors

A variety of mechanical and electrical mechanisms have been shown to tune sensory receptors to selected stimuli. This study demonstrates the presence of electrical tuning in the mammalian muscle spindle. Apamin and tetraethyl ammonium ions (TEA), blockers of Ca2+-activated K+ channels, and ZnCl2, a Ca2+ channel blocker, were shown to change the dynamic behavior of the muscle spindle. We conclude that a Ca2+-activated K+ channel in the sensory nerve of the spindle provides negative feedback which alters its dynamic behavior tending to compensate for muscle dynamics. This mechanism is similar to that found in sensory hair cells in the cochlea and vestibule and may represent a general strategy for tuning in sensory receptors.