Spontaneous and renal hypertensive rats: responsiveness of isolated right and left atria to noradrenaline, isoprenaline and methoxamine.

The adrenergic responsiveness of right and left atria isolated from spontaneously hypertensive rats (SHR) and renal hypertensive rats (RHR) was studied. Right atria isolated from SHR showed subsensitivity to the chronotropic effect of noradrenaline (21.3-fold at the EC50 level, P < 0.05) and isoprenaline (12.0-fold, P < 0.05). However, atria isolated from RHR did not exhibit any significant alteration (P > 0.05) in sensitivity to the chronotropic effect of noradrenaline or isoprenaline. Chronotropic responsiveness to theophylline was not altered in right atria isolated from SHR or RHR. alpha 1-Mediated inotropic responses to noradrenaline and methoxamine were blunted in left atria isolated from SHR. Left atria isolated from RHR showed supersensitivity to the inotropic effect of noradrenaline (5.4-fold at the EC50 level, P < 0.05) and subsensitivity to the inotropic effect of methoxamine (6.0-fold, P < 0.05). It is concluded that the present results could, at least partially, explain the reduced cardiac output observed during established hypertension in SHR and the increased cardiac output observed in the initial phase of renal hypertension in RHR.

Generation of atrial natriuretic peptide (ANP) in perfused lungs of spontaneously hypertensive rats (SHR). Comparison to Wistar-Kyoto (WKY) and Wistar (W) rat strains.

1. Lungs can take up from the vasculature, circulating forms of atrial natriuretic peptide (Turrin and Gillis, 1986, 1987) and also to synthesize ANP. 2. The lung peptide directly delivered by lungs into the lung vasculature could play a role in the local water/electrolytic balance. 3. Using Spontaneously Hypertensive Rats (SHR), isogenic normotensive controls, the Wistar-Kyoto strain (WKY), and the regular Wistar strain as second control (W), and using a highly sensitive RIA, we measured the immunoreactive IR-ANP content of extracted plasma, lung homogenate and lung perfusate, since there are references of altered ANP levels in this kind of hypertension. 4. The IR-ANP measured in the lung vasculature effluent collected throughout 32 min of Krebs perfusion, was significantly different in all of the three analyzed strains (SHR > WKY > W). 5. The results support the idea of a local function for the peptide hormone directly delivered into the lung vasculature of SHR, which could represent a local adaptation to haemodynamics SHR characteristics besides a genetic characteristic distinguishing WKY from W strains.

Lesions of the hypothalamus and pituitary inhibit volume-expansion-induced release of atrial natriuretic peptide.

Expansion of the blood volume causes a release of atrial natriuretic peptide (ANP) that is believed to be important in induction of the subsequent natriuresis and diuresis which, in turn, acts to reduce the increase in blood volume. Since stimulation of the anteroventral portion of the third cerebral ventricle (AV3V) induced a rapid elevation of plasma ANP, whereas lesions of the AV3V were followed by a marked decline in plasma concentration of the peptide, we hypothesized that release of ANP from the brain ANP neuronal system might be important to the control of plasma ANP. The perikarya of the ANP-containing neurons are densely distributed in the AV3V and their axons project to the median eminence and neural lobe. To test the hypothesis that these neurons are involved in volume-expansion-induced ANP release, by using electrolysis we destroyed the AV3V, the site of the perikarya, in male rats. Other lesions were made in the median eminence and posterior pituitary, sites of termination of the axons of these neurons, and also hypophysectomy was performed in other animals. In conscious freely moving animals, volume expansion and stimulation of postulated sodium receptors in the hypothalamus were induced by injection of hypertonic NaCl solution [0.5 or 0.3 M NaCl; 2 ml/100 g (body weight)]. Volume expansion alone was induced with the same volume of an isotonic solution (NaCl or glucose). In the sham-operated rats, volume expansion with hypertonic or isotonic solutions caused equivalent rapid increases in plasma ANP that peaked at 5 min and returned nearly to control values by 15 min. Lesions caused a decrease in the initial levels of plasma ANP on comparison with values from the sham-operated rats, and each type of lesion induced a highly significant suppression of the response to volume expansion on testing 1-5 days after lesions were made. Because a common denominator of the lesions was elimination of the brain ANP neuronal system, these results suggest that the brain ANP plays an important role in the mediation of the release of ANP that occurs after volume expansion. Since the content of ANP in this system is much less than that in the atria, there must be a remarkable increase in synthesis and release of brain ANP associated with this stimulus. It is also possible that blockade of volume-expansion-induced release of other neurohypophyseal hormones, such as endothelin, may block release of ANP from atrial myocytes. It is probable that volume expansion detected by stretch of atrial and carotid-aortic baroreceptors causes afferent input to the brain ANP system, thereby causing increased release of the peptide from the median eminence and neural lobe. Our results emphasize the importance of brain ANP to the control of ANP release to the blood.

Blockade of volume expansion-induced release of atrial natriuretic peptide by median eminence lesions in the rat.

Since stimulation of the anteroventral third ventricle region (AV3V) induced a rapid elevation of plasma atrial natriuretic peptide (ANP) associated with rapid changes in brain and pituitary content of ANP, whereas lesions of the AV3V were followed by a marked decline in plasma, brain and pituitary content of the peptide, we hypothesized that release of ANP from the median eminence (ME) might be an important pathway to control plasma ANP. Consequently, electrolytic lesions were placed in the ME and the response to hypertonic-volume expansion was determined in conscious rats. In sham-operated controls volume expansion produced a 3.5-fold increase in plasma ANP concentrations within 5 min. Values rapidly declined to near initial levels at 15 and 30 min. Median eminence lesions almost completely blocked the response to volume expansion at 24 and 120 h post-lesion and initial ANP concentrations were lower than those of the sham-operated controls. The results indicate that increased release of ANP from the neurohypophysis may play an important role in the increased plasma ANP concentrations which follow volume expansion.

Blockde of volume expansion-induced release of atrial natriuretic peptide by median eminence lesions in the rat

Since stimulation of the anteroventral third ventricle region (AV3V) induced a rapid elevation of plasma atrial natriuretic peptide (ANP) associated with rapid changes in brain and pituitary content of ANP, whereas lesions of the AV3V were followed by marked by a merked decline in plasma, brain and pituitary content of the peptide, we hypothesized that release of ANP from the median eminence (ME) might be an important pathway to control plasma ANP. Consequently, electrolytic lesions were placed in the ME and the response to hypertonic-expansion was determined in conscieous rats. In sham-operated controls volume expansion produced a 3.5-fold increase in plasma ANP concentrations within 5 min. Values rapidly declined to enar initial levels at 15 and 30 min. Median eminence lesions almost completely blocked the response to volume expansion at 24 and 120 h post-lesion and initial anp concentrations were lower than those of the sham-operated controls. The results indicate that increased release of ANP from the neurohypophysis may play an important role in the increased plasma ANP concentrations whic follow volume expansion

Role of the hypothalamus in the control of atrial natriuretic peptide release.

Stimulation of the region antero-ventral to the third cerebral ventricle (AV3V) by a cholinergic drug, carbachol, and lesions of the AV3V have been demonstrated in previous studies to either augment or decrease sodium excretion, respectively. Atrial natriuretic peptide (ANP) dramatically increases renal sodium excretion and has been localized to brain areas previously shown to be involved in control of sodium excretion. Consequently, to evaluate a possible role of brain ANP in evoking the changes in renal sodium excretion that follow stimulations or lesions of the AV3V, we determined the effect of injection of carbachol into the AV3V of rats on the concentration of plasma ANP and its content in several neural tissues, the pituitary gland, lungs, and atria. Conversely, the effect of lesions in the AV3V on plasma ANP and the content of the polypeptide in the various organs was determined. Injection of carbachol into the AV3V produced the expected natriuresis, which was accompanied within 20 min by a dramatic rise in the plasma ANP concentration and a rise in ANP content in the medial basal hypothalamus, the neurohypophysis, and particularly the anterior hypophysis but without alterations in the content of ANP in the lungs or the right or left atrium. Conversely, there was a dramatic decline in plasma ANP at both 24 and 120 hr after the AV3V lesions had been placed. This was accompanied by a slight decline in the content of the peptide in the lungs. There was no change in its content in the right atrium at 24 hr after lesions, but there was a significant increase at 120 hr. There was a small decline in the content in the left atrium at 24 hr, followed by a rebound to slightly elevated levels at 120 hr. These small changes contrasted sharply with the dramatic decline in content of the peptide in the medial basal hypothalamus, median eminence, neurohypophysis, choroid plexus, anterior hypophysis, and olfactory bulb. These declines persisted or became greater at 120 hr; except in the olfactory bulb in which the decline was no longer significant. The dramatic increase in plasma ANP after carbachol stimulation of the AV3V that was accompanied by marked elevations in content of the peptide in basal hypothalamus and neuro- and adenohypophysis suggests that the natriuresis resulting from this stimulation is brought about at least in part by release of ANP from the brain. Conversely, the dramatic decline in plasma ANP after AV3V lesions was accompanied by very dramatic declines in content of ANP in these same structures, which suggests that the previously shown decrease in sodium excretion obtained after these lesions may be at least in part due to a decrease in release of ANP from the brain. In view of the much larger quantities of the peptide stored in the atria, it is still possible that changes in atrial release may contribute to the alterations in plasma ANP observed after stimulation or ablation of the AV3V region; however, these results suggest that the dramatic changes in plasma ANP that followed these manipulations may be due to altered release of the peptide from brain structures as well as the atria and lungs.

Functional model for calcium mobilization in the depolarized isolated guinea-pig taenia coli.

The muscular response to pharmacological agents is directly dependent on intracellular ionic calcium (Ca2+) and is modulated by membrane permeability and cation-binding to subcellular organelles. The isometric contraction of the ventral guinea-pig taenia coli was registered in a bath with a Rich KCl-Ringer in the absence of Ca2+ and sodium (Na+) with or without EDTA or Sammarium (Sm3+). Our experimental sequence was based on alternate succession between the concentration-response-curves or decay-curves procedure to acetylcholine and barium (Ba2+) in the same preparation. It was found that a contracture elicited by barium ion was not significantly affected by inclusion of EDTA (.01 mM) or Sm3+ (.06 mM). Also, prior exposure of the depolarized muscle to acetylcholine did not affect the barium response. However, prior exposure of the muscle to acetylcholine sensitized the barium-induced contracture to the inhibitory actions of EDTA and Sm3+. Prior exposure of the muscle to barium ions strongly reduced the mechanical response to acetylcholine. In addition, tension recorded during an acetylcholine decay curve was observed to be reduced by EDTA and Sm3+, although some degree of tension could be elicited by acetylcholine even after the depolarized muscle had been exposed to EDTA or Sm3+ up to 70 min. The suggestion is put forward that barium ions mobilize intracellular stores of calcium which may be restored from calcium pools that are mobilized by acetylcholine, EDTA and Sm3+. Acetylcholine would be able to mobilize calcium ions from a firmly bound membrane store and additional less firmly bound membrane stores.(ABSTRACT TRUNCATED AT 250 WORDS)

Removal and release of recirculating atrial natriuretic peptide (ANP) by rabbit lungs perfused in situ.

The finding that radioiodinated, synthetic ANP (125I-ANP), analogous to the ANP secreted by the right atrium, binds to specific sites of endothelial cells, smooth muscle, adrenals and lungs (1-3) raised the importance of analyzing lung-ANP interactions. Rabbit lungs were accordingly loaded with 125I-ANP, 2000 Ci/mM (2.5 pM) in Krebs-albumin recirculating perfusion medium at a rate of 20 ml/min, over 27 min. 125I-ANP binding was estimated by taking samples at 2-min intervals from the perfusate. Fourteen min after recirculation, uptake was maximal (66.4 +/- 5.2%, N = 8) (X +/- SEM) and not different from that observed after a single passage through the lungs and was dose-dependent, antagonized by unlabelled ANP. The system was then changed to single-pass washout and efflux curves were obtained, collecting the entire efflux every 6 s per sample, over 15 min. Efflux curves after 125I-ANP loading fitted a bi-exponential decay model (Y(to) = Y(t)e-b11t + Y(t)e-b2t). Lung ANP uptake/release may be involved in regulating systemic concentrations and hence renal and other actions of ANP.

Removal and release of recirculating atrial natriuretic peptide (ANP) by rabbit lungs perfused in situ

The finding that radioiodinated, synthetic ANP (125I-ANP), analogous to the ANP secreted by the right atrium, binds to spepcific sites odf endothelial cells, smooth muscle, adrenals and lungs (1-3) raised the importance of anlyzing lung-ANP interactions. Rabbit lungs were accordingly loaded with 125I-ANP, 2000 Ci/mM (2.5 pM) in Krebs-albumin recirculating perfusion medium at a rate of 20 ml/min, over 27 min. 125 I-ANP binding was estimated by taking samples at 2-min intervals from the perfusate. Fourteen min after recirculation, uptake was maximal (66.4 ñ 5.2%, N = 8) (X ñ SEM) and not different from that observed after a single passage through the lungs and was dose-dependent, antagonized by unlabelled ANP. The system was then changed to single-pass washout and efflux curves were obtained, collecting the entire efflux every 6 s per sample, over 15 min. Efflux curves after 125I-ANP loading fitted a bi-exponential decay model (Y(to) = Y(t)e -b1t+ Y(t)e -b2t). Lung ANP uptake/ release may be involved in regulatinh systemic concentrations and hence renal and other actions of ANP

Samarium as a tool in studies on the responsiveness of the isolated guinea pig ventral taenia coli to barium.

The responsiveness of isolated guinea pig ventral taenia coli to barium in polarized (Ringer-Tris medium) and depolarized (Ringer-KCl medium) preparations was assayed in the presence of samarium (Sm3+, 6 or 8 X 10(5) M). No difference in EC50 or maximal response was observed between polarized and depolarized preparations (p greater than 0.05). The lower Sm3+ concentration increased Ba2+ EC50 in both experimental conditions (p less than 0.05) although no alteration was observed as to the maximal response (p greater than 0.05). On the other hand, maximal tension was strongly reduced by the higher Sm3+ concentration in polarized preparations (p less than 0.01) while a potentiation of the maximum response was observed on the depolarized tissues (p less than 0.05). A possible role for calcium ions in the mediation of the effects is suggested.