Sham-operated Guinea Pigs Research Articles

Involvement of neuroexcitatory amino acids in the control of gonadotropin-releasing hormone release from the hypothalamus of the adult male guinea pig: predominantly inhibitory action of N-methyl-D-aspartate-mediated neurotransmission and its reversal after orchidectomy.

Excitatory amino acids (EAA) have been implicated in the control of LH secretion through facilitation of GnRH release, although disparate findings of an inhibitory effect of EAA on LH secretion in several species after gonadectomy have been reported. Using a static incubation system, we studied the effects of EAA receptor agonists and antagonists on in vitro GnRH release from the isolated medial basal hypothalamus (MBH) of the male guinea pig. In the presence of 0.4 mM glycine, the N-methyl-D-aspartate (NMDA)-specific receptor agonist N-methyl-D,L-aspartic acid (NMA) exerted a dose-dependent inhibition of GnRH output from the MBH of intact guinea pigs, which was significant (P < or = 0.01) at concentrations of 1 and 50 mM. The NMDA-specific receptor antagonist D,L-2-amino-5-phosphonovaleric acid (AP-5) significantly stimulated GnRH release at 10(-3) and 1 mM (P < or = 0.001) and was ineffective at 10(-6) mM. NMA (50 mM) and AP-5 (1 mM) produced similar effects when tested at a lower glycine concentration of 10 nM. The presence of the two compounds together resulted in overall unchanged GnRH output. The inhibitory effect of 50 mM NMA was also effectively blocked in the presence of 2 mM Mg2+ at both high and low glycine concentrations. When, for comparative purposes, isolated MBHs of 50-day-old intact male rats were exposed to 50 mM NMA, a response opposite to that seen in the intact guinea pig was observed, with a marked increase in GnRH output (P < or = 0.001). When tested at 10 nM glycine, 50 mM kainic acid, a non-NMDA-specific receptor agonist, on the other hand, had a marked stimulatory effect on GnRH output (P < or = 0.01) from intact guinea pig MBHs, an action that was prevented in the presence of the kainate/quisqualate receptor antagonist 6,7-dinitro-quinoxaline-2,3-dione (0.1 mM); 6,7-dinitro-quinoxaline-2,3-dione alone inhibited GnRH release (P < or = 0.01). In a separate series of experiments, the effects of EAA (in the presence of 10 nM glycine) on GnRH release from the MBHs of long term orchidectomized and sham-operated guinea pigs were compared. Orchidectomy led to a dramatic reversal of the NMDA-mediated inhibition of GnRH secretion observed in MBHs of sham-operated animals, with 50 mM NMA producing a marked increase (P < or = 0.0001) and 1 mM AP-5 resulting in a clear inhibition (P < or = 0.0001) of GnRH release. Kainic acid (50 mM), on the other hand, had a similar stimulatory action on GnRH release from the MBHs of both orchidectomized (P < or = 0.0001) and sham-castrated (P < or = 0.001) guinea pigs.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in cell length consequent on depolarization in single left ventricular myocytes from guinea-pigs with pressure-overload left ventricular hypertrophy

Cell length was measured in single guinea-pig left ventricular myocytes by using a high-resolution photodiode array. Step depolarizations from a holding potential of -45 mV were applied using a switch-clamp technique with 2 M KCl microelectrodes, which were devoid of Ca2+ buffering. Comparison was made between myocytes from sham-operated guinea-pigs and guinea-pigs with mild pressure-overload left ventricular hypertrophy induced by infra-renal aortic constriction. The relation between cell shortening and membrane voltage was bell shaped, and a phasic component of shortening was evident at the range of potentials over which the L-type calcium current was activated. Mean cell shortening was increased in the hypertrophy group, and was maximal at +15 mV in both groups (control, 7.6 +/- 0.9 microns, n = 11, hypertrophy 11.0 +/- 1.2 microns, n = 20, p < 0.05). The latency to the onset of contraction was significantly shorter in the hypertrophy myocytes at -25 mV and at potentials positive to +50 mV. The relation between time-to-peak shortening and voltage showed a trend to shorter times in the hypertrophy group. At very positive potentials a slow component of contraction was identified which was relatively larger in the hypertrophy myocytes. This finding is consistent with increased calcium entry via sarcolemmal sodium-calcium exchange in the myocytes from the hypertrophy group.

Diminished neuropeptide Y and dopamine beta-hydroxylase immunoreactivity in a guinea pig model of left ventricular hypertrophy.

The aims of the study were to determine the effect of chronic pressure overload of the left ventricle on the density and distribution of neuropeptide-Y-like immunoreactive (NPY-LI) nerve fibres in heart and to compare any changes to those observed in adrenergic nerve fibres, identified by dopamine beta-hydroxylase immunoreactivity. Pressure overload was produced in female adult guinea pigs by constriction of the abdominal aorta, using a modified Weck haemoclip. The same operation was performed on a separate group of animals except that no clip was placed around the aorta. Five weeks after surgery, animals were anaesthetised, and the hearts were fixed by perfusion for immunohistochemistry. Cryostat sections were stained, using an indirect peroxidase/antiperoxidase method, for NPY or dopamine beta-hydroxylase. Aortic stenosis caused a 45% increase in left ventricular weight and a 58% increase in left atrial weight at 5 weeks postsurgery. Pulmonary oedema, a sign of cardiac failure, was evident in most of the animals with aortic stenosis. Immunohistochemical studies showed that in atria and right ventricles from animals with abdominal aortic stenosis the distribution and density of NPY-LI nerve fibres were similar to those in the sham operated guinea pigs. However, the left ventricles obtained from the animals with aortic stenosis were nearly devoid of NPY-LI nerve fibres. The density of dopamine beta-hydroxylase-LI nerve fibres was also substantially reduced in the hypertrophied left ventricles. Aortic stenosis resulting in left ventricular hypertrophy caused a nearly complete loss of NPY-LI and dopamine beta-hydroxylase-LI nerve fibres from the left ventricle. The parallel reduction in both neuropeptide Y and dopamine beta-hydroxylase is in accordance with the association of neuropeptide Y with sympathetic (adrenergic) nerve fibres in the left ventricle and suggests that chronic left ventricular hypertrophy causes a severe degeneration of sympathetic axons supplying this chamber and/or reduces the ability of these sympathetic neurones to maintain normal levels of neurotransmitter related enzymes and neuropeptides.

Ventricular hypertrophy and presynaptic regulation of sympathetic function

In normal heart, presynaptic cholinergic muscarinic and alpha 2-adrenergic mechanisms contribute to regional variations in the rate constant of norepinephrine turnover (kNE), an index of sympathetic neural function. To evaluate these mechanisms in the hypertrophied heart, pulmonary artery-constricted and sham-operated guinea pigs were pretreated with 1) saline vehicle (control) or 2) a combination of quinuclidinyl benzilate (Q), a muscarinic cholinergic antagonist, and yohimbine (Y), an alpha 2-adrenergic antagonist. An increase in kNE was determined in multiple regions of heart from incorporation of radiolabeled tyrosine into norepinephrine during a control period at 24 degrees C and again at 4 degrees C. In sham animals, kNE during cold stress was increased significantly (P less than 0.05) by Q + Y compared with vehicle, confirming that muscarinic cholinergic and/or alpha 2-adrenergic receptors exert a negative-feedback influence on sympathetic neurotransmitter synthesis. In pulmonary artery-constricted animals, in contrast, there were smaller increases in cardiac kNE compared with sham guinea pigs given Q + Y and subjected to cold stress. These data support the concept that muscarinic cholinergic and/or alpha 2-adrenergic presynaptic regulation of cardiac sympathetic function is altered in the hearts and vasculature of pulmonary artery-constricted guinea pigs.

Volume receptors in guinea pig labyrinth: relevance with respect to ADH and Na control.

We tested the hypothesis that changes in extracellular fluid volume are reflected by pressure changes within structures of the inner ear and that through neural pathways, a control mechanism exerts an influence on antidiuretic hormone (ADH) release and Na excretion. The study was performed on 35 guinea pigs. In protocol 1, 13 animals were studied before and after decompression of the inner ear by bilateral fluid withdrawal in an experimental setting of sustained isotonic expansion that kept the osmoreceptor partially activated and the intrathoracic volume receptors suppressed. A group of six sham-operated animals served as control. In protocol 2, nine animals were studied before and after a unilateral rise in their inner ear pressure during slightly hypertonic low-rate infusions that kept the osmoreceptor and thoracic volume receptors stimulated. A group of seven sham-operated guinea pigs served as controls. Decompression of the inner ear was attended by a rise in plasma ADH from 11.9 +/- 2.4 to 29.1 +/- 6.9 pg/ml, in urine osmolality (Uosmol) from 470 +/- 48 to 712 +/- 46 mosmol/kg (P less than 0.001), and a fall in urine flow rate (V) from 184 +/- 47 to 71 +/- 11 microliters/min (P less than 0.01), whereas plasma Na (PNa) and osmolality (Posmol) did not change. During inner ear hypertension, plasma ADH fell from 25.6 +/- 3.9 to 18.4 +/- 3.1, Uosmol from 829 +/- 58 to 627 +/- 43 (P less than 0.001), and V rose from 51 +/- 11 to 130 +/- 23 (P less than 0.001), whereas glomerular filtration rate, PNa, and Posmol did not change.(ABSTRACT TRUNCATED AT 250 WORDS)

Response of the renin-angiotensin system to relief of neonatal ureteral obstruction.

Chronic partial ureteral obstruction (CPUO) causes an increase in renal vascular resistance (RVR) that can be prevented by angiotensin II (ANG II)-converting enzyme inhibition. To assess the early effects of CPUO, guinea pigs subjected to neonatal left ureteral obstruction were anesthetized at 11-13 days of age for measurement of cardiac output and renal blood flow (RBF) using radioactive microspheres. Although RBF of the hydronephrotic kidney was not decreased at this age, that of the intact kidney more than doubled (P less than 0.01). To evaluate the hemodynamic response to relief of 5 or 10 days of neonatal left CPUO, animals were studied at 19-28 days of age. Although chronic enalapril maleate administration (30 mg.kg-1.day-1) did not further reduce the ratio of RVR to total vascular resistance (TVR) of the postobstructed kidney, it lowered RVR/TVR of the intact contralateral kidney by 40% (P less than 0.05). Renal renin content [RRC, pg angiotensin I (ANG I).mg protein-1.h-1] was twofold higher in both kidneys of animals with unilateral CPUO compared with those of sham-operated guinea pigs (P less than 0.03), and relief of obstruction normalized RRC. The rise in RVR/TVR resulting from ANG II infusion was not different in left kidneys of sham, CPUO, and CPUO-relief groups. However, for the intact kidney of animals with contralateral relief of CPUO, the increase was greater than in remaining groups (P less than 0.05). We conclude that by reducing intrarenal renin, relief of neonatal unilateral CPUO decreased ANG-mediated vasoconstriction of the postobstructed kidney and increased the vasoconstrictor response of the intact kidney to ANG II.

Effects of anterior hypothalamic lesions and sham-operations on bacterial endotoxin-induced non-specific airway hyperreactivity in vivo and in vitro.

1. In the present study the effects of anterior hypothalamic (AHA) lesions and sham-operations were investigated on the endotoxin-induced airway hyperreactivity in guinea-pigs. Unoperated, sham-operated and AHA-lesioned guinea-pigs were injected intraperitoneally with Escherichia coli endotoxin and the airway reactivity tested four days later in isolated tracheal spirals and in spontaneously breathing anaesthetized animals. Control animals were given sterile saline. 2. Sham-operated control animals demonstrated a diminished responsiveness of the tracheal spirals in vitro and of the lung resistance (delta R1) in vivo to histamine receptor and cholinoceptor-muscarinic agonists as compared to unoperated control animals. 3. AHA-lesioned control animals showed a responsiveness of the respiratory airways in vitro and in vivo between the values of unoperated and sham-operated control animals, suggesting that lesions partially restored the diminished responsiveness. 4. In unoperated and sham-operated guinea-pigs, endotoxin administration induced hyperreactivity of the tracheal spirals and delta R1 to histamine receptor and cholinoceptor-muscarinic agonists with respect to the control groups. 5. In AHA-lesioned animals, the endotoxin-induced airway hyperreactivity in vitro and in vivo to histamine receptor and cholinoceptor-muscarinic agonists was absent.

Suppression of fever after lesions of the anteroventral third ventricle in guinea pigs

Endogenous pyrogen (EP), injected systemically or intracerebrally, evokes fever and certain changes in plasma trace metal and glycoprotein levels which are characteristic of the acute-phase reaction. It is generally assumed that EP enters the brain from the blood, although it has not yet been demonstrated that EP crosses the blood-brain barrier (BBB). The possibility that EP might penetrate the brain through the organum vasculosum laminae terminalis (OVLT), which is outside of the BBB and located in close proximity to the medial preoptic region (MPO, the primary site sensitive to locally applied EP), was investigated by making electrolytic lesions (3 mA, 20 sec, anodal) in the anteroventral wall of the third ventricle of guinea pigs (AV3V-X). After 10 days, their febrile and selected acute-phase responses (plasma iron, zinc, copper, and sialic acid levels) to endotoxin (LPS, S. enteritidis, 2 μg/kg, IP), which induces EP production by the host, were measured; controls were sham-operated guinea pigs. LPS did not induce in the AV3V-X animals either fever or rises in plasma copper and sialic acid levels; however, as in the controls, it caused hypoferremia and hypozincemia. To exclude damage to the MPO as a cause of these responses, sham and AV3V-X guinea pigs were administered homologous EP intrapreoptically (1 μ1 bilaterally). Comparable fevers developed in both groups of animals. Hence, the integrity of the AV3V region including the OVLT seems to be critical for the EP-induced elevations of both body temperature and plasma levels of acute-phase proteins, but not for the fall of plasma iron and zinc levels. It may be that EP passes into the brain through the OVLT.

Hemodynamic adaptation to reduced renal mass in early postnatal development.

Radioactive microspheres, clearance methodology, and glomerular counting techniques were used to compare hemodynamic changes resulting from uninephrectomy at birth with those of sham-operated guinea pigs 8-13 days of age and 18-24 days old. Left renal blood flow doubled from 10-20 days of age, and was approximately 65% higher in animals with reduced renal mass. Cardiac output also doubled from 10-20 days of age but was not significantly affected by uninephrectomy whereas mean arterial blood pressure rose with normal or compensatory renal growth. There was a progressive fall in renal vascular resistance that paralleled the drop in total peripheral resistance during normal growth, but the fraction of cardiac output supplying the remaining kidney of renoprival guinea pigs rose significantly from 7% at 10 days to 11% at 20 days of age. Whole kidney filtration fraction averaged 0.25-0.29 for all groups. There was no change in cortical blood flow distribution with age or after uninephrectomy: 50, 30, and 20% supplied outer, middle, and inner thirds, respectively. Glomerular perfusion rate increased proportionately in all cortical levels during normal or compensatory renal growth, and was higher in outer and inner cortical thirds than in the middle cortex. It is concluded that systemic and intrarenal hemodynamic responses to uninephrectomy at birth are similar to the pattern of normal maturation. Reduction in renal mass and normal somatic growth provide additive stimuli resulting in maintenance of homeostasis.

Glomerular number and perfusion during normal and compensatory renal growth in the guinea pig.

Changes in glomerular number and perfusion during maturation of the guinea pig were compared with the response to uninephrectomy at birth or 84 days of age. From birth to adulthood, there was a 21% increase in number of glomeruli identified by the presence of India ink previously injected in vivo. For animals uninephrectomized at birth, a similar increase occurred several wk earlier than in sham-operated littermates; however, uninephrectomy in adulthood resulted in no further increment. Although the number of glomeruli in the left kidney varied by 25% within each group, the difference in number between right and left kidneys of individual animals averaged only 4%. Additional glomeruli not identified by India ink were subsequently revealed by application of Wright's stain. These comprised 13-17% of the glomeruli containing India ink in 1 and 22-day-old sham groups, but only 2% in 22-day-old uninephrectomized or adult animals. Approximately 3/4 of all glomeruli identified by India ink were present in the outer cortex of 22-day-old uninephrectomized and sham-operated guinea pigs; the cortical distribution of Wright-stained glomeruli in the latter group was similar. Paraffin sections of kidneys revealed scattered glomeruli not containing India ink in 1-day-old and 22-day-old sham guinea pigs, but almost none in 22-day-old uninephrectomized animals. There were no glomeruli in an early stage of formation in any of the sections examined. It is concluded that during development, an increase in number of glomeruli identified by India ink represents nephrons which are underperfused in the neonate but are completely perfused at maturity. This process is accelerated by uninephrectomy in early development, but is unaffected by uninephrectomy in adulthood, at which time glomerular perfusion is virtually homogeneous.

Functional adaptation to reduced renal mass in early development.

To determine whether reduced renal mass in the newborn results in acceleration of normal renal development, the response to unilateral nephrectomy (N) before 36 h of age was compared with sham-operated (S) guinea pigs during the period of most rapid nephron maturation. Studies were performed at 7-13 days (group I) and 19-25 days (group II). Mean arterial blood pressure (AP), left kidney glomerular filtration rate (LKGFR), and urine sodium excretion (UNaV) were measured. Superficial single nephron GFR (sSNGFR) and proximal fractional water reabsorption (FRH2O) were measured by micropuncture, and the number of glomeruli (NG) was determined by India ink perfusion. In view of the susceptibility of the neonate to extracellular fluid loss, groups I and II were plasma infused to maintain euvolemia and group II was compared with 19- to 25-day-old hydropenic animals (group III). Increase in body weight with age was unaffected by neonatal N. In group IN, the compensatory increase in sSNGFR was greater than SNGFR for deeper nephrons, which normally contribute most to GFR at this age. In group IIN there was an 80% adaptive increase in LKGFR that could not be entirely explained by the rise in SNGFR. Since NG in group IIN was greater than in group IIS and similar to that in adulthood, the enhanced adaptation in LKGFR in group IIN may be due in part to earlier recruitment of a population of underperfused glomeruli. FRH2O did not change significantly with age and did not differ in N and S groups. Animals in group III developed a rise in hematocrit during the experiment, and AP, LKGFR, and UNaV were lower in group IIIN than in group IIN. It is concluded that following N at birth, the sequence of renal functional maturation is accelerated while glomerulotubular balance is preserved. As a result of these adaptative changes, homeostasis is maintained and body growth proceeds without impairment.

Chronic heart failure in the guinea pig increases cardiac α 1- and β-adrenoceptors

Compared with sham-operated guinea pigs, chronic heart failure produced by aortic constriction resulted in a 79% increase in α 1-adrenoceptor density as measured by [ 3H]prazosin binding (73 ± 15 vs. 131 ± 20 fmol/mg protein, P < 0.05); β-adrenoceptor number, measured by (−)-[ 3H]dihydroalprenolol binding, increased by 93% (28 ± 3.4 vs. 54 ± 5.2 fmol/mg protein, P < 0.01). For each radioligand K D values were unchanged. Thus, congestive heart failure due to chronic pressure overload results in compensatory augmentation of α 1- and β-adrenoceptor numbers.

Effect of hypothalamic lesions on immediate hypersensitivity.

Bilateral electrolytic lesions were placed in the anterior basal hypothalamus of male Hartley-strain guinea pigs. The animals with anterior hypothalamic lesions and a group of sham-operated controls were sensitized 5-10 days postoperatively with ovalbumin in modified Freund's adjuvant. At 18-22 days following sensitization both groups were randomly divided into three subgroups and challenged by intrajugular injections of three different antigen doses. Anaphylactic death and the severity of anaphylaxis in the survivors were recorded. Antibody titers were determined prior to challenge with use of the passive cutaneous anaphylaxis technique. Serial sections of each brain were made for lesion localization. Significant protection against anaphylaxis was found in guina pigs with symmetrical lesions in the anterior hypothalamus. No significant differences in antibody titers were observed between the lesion and sham-operated groups. There were no significant differences in the response of isolated muscle strips from anterior-lesioned and sham-operated guinea pigs to exogenous histamine. Anterior hypothalamic lesions of size and location comparable to those that provided protection against lethal anaphylaxis did not modify the anaphylactic response of isolated ileum from actively sensitized animals or following passive sensitization in vitro.

Effect of a intrauterine foreign body during the estrous cycle on plasma progesterone in guinea pigs.

SummaryThe progesterone concentration in peripheral plasma of sham-operated guinea pigs increased significantly during the estrous cycle from a nondetectable concentration at estrus to a maximum concentration on day 10, followed by a rapid decline. In the presence of an IUFB in both uterine horns, the progesterone concentration did not increase after day 4 of the cycle. Over the next 8 days, a slow decline in concentration occurred. The length of the estrous cycle was significantly shorter in animals having IUFB than in sham-operated animals.