[Diabetic cardiomyopathy: pathophysiological mechanisms of structural and functional changes.] / Diabetická kardiomyopatie: patofyziologické mechanismy strukturnich a funkenich zmén.

Diabetes mellitus is a powerful risk factor for cardiovascular disease associated with high morbidity and mortality rates. Diabetic patients also have an increased incidence of heart failure which has been traditionally attributed to the presence of ischemic or hypertensive heart disease. However, the diabetic milieu is itself noxious to the heart, and cardiomyopathy can develop independent of elevated blood pressure, coronary artery disease or other risk factors with the potential to lead to a progressive development of heart failure. Diabetic car- diomyopathy is characterized by significant changes in function and structure of the heart. They have been studied in numerous diabetic experimental models in animals, mostly rodents. Revealing of potential underlying mechanisms of these pathophysiological alterations holds the promise to design new pharmacological strategies for diabetic patients. Our current review provides an update on functional and structural alterations in the diabetic heart and pathophysiological mechanisms of their development and progression.

[Adrenergic regulation of the mammalian heart]. / Adrenergní regulace srdecní cinnosti savcu.

Adrenergic system in the mammalian heart plays a pivotal role in regulation of contractility and/or heart rate. At present, nine subtypes of adrenergic receptors (AR) have been identified. Among these there are six AR localized in the plasma membrane of cardiomyocytes. They mediate their effects by increases in the intracellular level of various signaling molecules which initiate diverse cellular responses. The effects of stimulation of both beta-AR by catecholamines noradrenaline and adrenaline are consistent with coupling to the Gs protein-adenylyl cyclase-cAMP-protein kinase classical pathway, with consequent protein kinase A-catalysed phosphorylation of target enzymes responsible for increased contractility and hastening of relaxation. In contrast to beta1-AR, beta2-AR can also couple to G(i) protein which causes cAMP-independent control of calcium signaling and contraction. Activation of beta-AR obviously couples to a G(i)/ nitric oxide pathway and mediates a decrease in contractile force, whereas stimulation of alpha-AR increases contractility via G protein/phospholipase C/diacylglycerol/inositoltrisphosphate/protein kinase C pathway. These findings reveal the diversity and specifity of AR subtypes and G protein interactions. They also provide new insights in understanding the differential regulation and functionality of AR subtypes in healthy and diseased hearts.

Expression of neuropeptide Y and its receptors Y1 and Y2 in the rat heart and its supplying autonomic and spinal sensory ganglia in experimentally induced diabetes.

Diabetic cardiomyopathy, involving both cardiomyocytes and the sensory and autonomic cardiac innervation, is a major life-threatening complication in diabetes mellitus. Here, we induced long-term (26-53 weeks) diabetes in rats by streptozotocin injection and analyzed the major cardiac neuropeptide signaling system, neuropeptide Y (NPY) and its receptors Y1R and Y2R. Heart compartments and ganglia supplying sympathetic (stellate ganglion) and spinal sensory fibers (upper thoracic dorsal root ganglia=DRG) were analyzed separately by real-time reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry. Ventricular, but not atrial innervation density by NPY-immunoreactive fibers was diminished, and preproNPY expression was transiently (26 weeks) reduced in left atria, but remained unchanged in sympathetic neurons and was not induced in DRG neurons. In all ganglia and heart compartments, Y1R expression dominated over Y2R, and Y1R-immunoreactivity was observed on cardiomyocytes and neuronal perikarya. Atrial, but not ventricular Y1R expression was up-regulated after 1 year of diabetes. Collectively, these data show that a disturbance of the cardiac NPY-Y1R/Y2R signaling system develops slowly in the course of experimentally induced diabetes and differentially affects atria and ventricles. This is in parallel with the clinically observed imbalances of the cardiac autonomic innervation in diabetic cardiac autonomic neuropathy.

Depressed cardiac contractility and its postnatal development in rats after chemical sympathectomy.

The contribution of the sympathetic innervation to the postnatal development of cardiac contractility remains unclear. In this study, the postnatal maturation of cardiac contractility was compared in control rats and rats after chemical sympathectomy. The chemical sympathectomy was induced by administration of 6-hydroxydopamine to newborn rats. At days 20, 40 and 60 of postnatal life, the contractile parameters and concentrations of sympathetic neurotransmitters were measured in both right and left ventricles. In rats with chemical sympathectomy, concentrations of norepinephrine were reduced almost completely in both ventricles at all time points. The contractility of the left ventricle papillary muscles was substantially decreased at all time points. In contrast, the contractility of the right ventricle papillary muscles was decreased only transiently, showing a recovery at day 60 regardless of the permanently decreased concentration of norepinephrine. The concentration of neuropeptide Y, another neurotransmitter present in sympathetic nerves, showed the same developmental trend as contractility: permanent reduction in the left ventricle, transient reduction with a recovery at day 60 in the right ventricle. The data indicate that the sympathetic nervous system plays an important role in the postnatal development of cardiac contractility and neuropeptide Y may contribute to this effect.

Chronic atropine administration diminishes the contribution of vasoactive intestinal polypeptide to heart rate regulation.

Vasoactive intestinal polypeptide (VIP) is implicated in the modulation of vagal effects on the heart rate. In this study, the impact of acute and chronic atropine administration on VIP levels in rat heart atria was investigated in relation to heart rate in the course of vagus nerves stimulation. Anaesthetised control and atropinised (10 mg/kg/day for 10 days) rats pretreated with metipranolol and phentolamine that were either given or not a single dose of atropine were subjected to bilateral vagus nerve stimulation (30 min: 0.7 mA, 20 Hz, 0.2 ms). VIP concentrations in the atria were determined after each stimulation protocol. In control rats with or without single atropine administration, the heart rate upon vagal stimulation was higher than in atropinised animals with or without single atropine dose, respectively. VIP concentrations in the control atria were significantly decreased after the stimulation; the decrease was comparable both in the absence and presence of a single dose of atropine. Compared to controls, VIP levels were significantly decreased after chronic atropine treatment and they were not further reduced by vagal stimulation and single atropine administration. Administration of VIP antagonist completely abolished the differences in the heart rate upon vagal stimulation between control and atropinised groups. In conclusion, the data indicate that chronic atropine administration affects VIP synthesis in rat heart atria and consequently it modifies the heart rate regulation.

IP3 type 1 receptors in the heart: their predominance in atrial walls with ganglion cells.

Previously we have shown that inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) are abundantly expressed in the atria of rat hearts. Since arrangement of atria is very heterogeneous, in this work we focused on the precise localization of IP3 receptors in the left atrium, where the gene expression of the type 1 IP3R was the highest. The mRNA levels of the IP3 type 1 receptors in the left atrium, left ventricle and myocytes were determined using real-time polymerase chain reaction and Taqman probe. For precise localization, immunohistochemistry with the antibody against type 1 IP3Rs was performed. The mRNA of type 1 IP3 receptor was more than three times higher in the left atrium than in the left ventricle, as determined by real-time PCR. Expression of the type 1 IP3 receptor mRNA was higher in the atria, especially in parts containing cardiac ganglion cells. The atrial auricles, which are particularly free of ganglion cells, and the ventricles (wall of the right and left ventricle and ventricular septum) contained four to five times less IP3 receptors than atrial samples with ganglia. IP3R type 1 immunoreactivity detected by a confocal microscope attributed the most condensed signal on ganglionic cells, although light immunoreactivity was also seen in cardiomyocytes. These results show that type 1IP3 receptors predominate in intrinsic neuronal ganglia of cardiac atria.

Cardiomyopathy in streptozotocin-induced diabetes involves intra-axonal accumulation of calcitonin gene-related peptide and altered expression of its receptor in rats.

Calcitonin gene-related peptide (CGRP) is a vasorelaxant and positive inotropic and chronotropic peptide that binds to the calcitonin receptor-like receptor. In the heart, upon stimulation CGRP is released from sensory nerve terminals and improves cardiac perfusion and function. In the present study, we investigated alterations in the components of the CGRP signaling system during development of diabetic cardiomyopathy. Rats received a single injection of streptozotocin. Four, 8, and 16 weeks thereafter cardiac CGRP content (radioimmunoassay), calcitonin receptor-like receptor expression (by real-time RT-PCR), and CGRP and calcitonin receptor-like receptor tissue distribution (immunohistochemistry) were assessed. CGRP content of atria and ventricles progressively increased during the 4 months following streptozotocin-treatment, while the distribution of CGRP-immunoreactive fibers was not visibly altered. Conversely, cardiac expression of calcitonin receptor-like receptor initially (4 weeks after treatment) increased but then gradually declined to 47% of control levels in both atria after 16 weeks. These quantitative changes were not associated with altered cellular distribution patterns (primarily in venous and capillary endothelium). Since sensory neurons have been reported to decrease expression of the CGRP precursor in the course of diabetes, the intra-axonal accumulation of CGRP observed here reflects impaired release, which, coupled with the down-regulation of its cognate receptor, calcitonin receptor-like receptor, may contribute to the well-documented impairment of cardioprotective functions in diabetes.

Norepinephrine release in the heart atria of diabetic rats.

The content, release and uptake of norepinephrine (NE) in the sympathetic nerves of the rat heart atria were studied in the course of diabetes and in age-matched controls. Diabetes was induced by streptozotocin (STZ) and rats were subjected to further experiments 1, 4 or 7 months later (STZ1, STZ4, STZ7). Isolated atria were superfused with oxygenated Krebs-Henseleit (KH) solution. After equilibration, four 10-min fractions were collected: B1, basal release of NE; S1, potassium-evoked release (KER), where NE outflow was stimulated by depolarisation with 50 mmol/l KCl; B2, basal release of NE under the influence of the neuronal uptake blocker desipramine (DES); S2, KER under the influence of DES. The content of NE was measured by radioimmunoassay. In STZ4 and STZ7 rats, NE concentrations were significantly lower in both atria compared to controls. B1 and S1 were significantly higher in STZ4 than in control atria. DES increased KER of NE in controls only. In contrast, DES caused a significant decrease in B2 and S2 in STZ4 atria, suggesting that a substantial portion of NE release was due to a calcium-independent carrier-mediated process. In experiments with calcium-free KH solution in fractions B2 and S2, KER ill controls was nearly abolished. However, in STZ4 and STZ7 atria, S2 was still significantly higher than B2. In conclusion, the NE-releasing mechanism may be different in the chronically diabetic animals than in healthy subjects and may contribute to the decreased NE concentration in the STZ atria.

Vasoactive intestinal polypeptide concentrations in heart atria of hypothyroid rats.

OBJECTIVES: To determine putative effects of various protocols of propylthiouracil (PTU)-induced hypothyroidism on vasoactive intestinal polypeptide-like immunoreactivity (VIP-LI) levels in the atria of developing and adult female rats. ANIMALS AND METHODS: Perinatal hypothyroidism was induced by treating pregnant rats with 0.05% PTU in drinking water from late gestation till the age of 60 days (P-PTU). Adult rats were given PTU for 10, 30 or 70 days (PTU-10, PTU-30 and PTU-70, respectively). Corresponding age-matched controls were left intact (P-Cont, Cont-10, Cont-30 and Cont-70, respectively). Resting heart rate, serum total thyroxine concentration, body weight and atrial weight were determined in all animals. VIP-LI levels in tissue extracts were measured by radioimmunoassay. RESULTS: The values of heart rate, serum total thyroxine, body weight and atrial weight showed that 10-day treatment did not suppress thyroid gland function completely. However, the remaining experimental protocols were sufficient to reach stable hypothyroid conditions. Thyroid hormone deficiency led to a significant increase in VIP-LI levels in both atria of PTU-30 and PTU-70 rats (P<0.01 versus corresponding controls). Interestingly, in P-PTU atria, VIP-LI reached significantly higher values than in rats treated with PTU for the same time during adulthood (PTU-70). CONCLUSIONS: These results provide new evidence that hypothyroidism interferes with VIP-ergic innervation in rat heart atria. The impact of thyroid hormone deficiency on VIP-LI levels differed in P-PTU and PTU-70 rats suggesting that thyroid hormone may play an important part in the development of VIP-ergic innervation in rat heart atria.

Vasoactive intestinal polypeptide in rat heart atria: the effect of hyperthyroidism.

The effects of transient and sustained hyperthyroidism on vasoactive intestinal polypeptide-like immunoreactivity (VIP-LI) levels were studied in the heart atria of developing and adult rats. Newborn rats were divided into 5 groups. Neo-T animals were treated with thyroxine (T4) during postnatal days 1-8 and sacrificed at the age of 60 days. Neo-S rats were treated with T4 during postnatal days 1-60 and sacrificed one day later. Adult-1 and Adult-2 animals received T4 during days 52-60 and were sacrificed 5-6 days and 1 day later, respectively. Control animals were injected with saline. VIP-LI concentrations were determined in extracts from the left and right atria separately. In Neo-S and Adult-2 rats, spontaneous heart rate, the weight of both atria and total T4 serum levels were significantly enhanced, while their body weight was decreased. The ratio atria weight to body weight was significantly increased in all groups except for Adult-1 animals. Hyperthyroidism led to a significant decrease in VIP-LI levels in both atria of Neo-S and Neo-T rats. Hyperthyroidism induced in adult rats also decreased VIP-LI levels in both atria. However, this change was only transient. In conclusion, our data have provided new evidence that hyperthyroidism induced during the early neonatal period interferes with the development of VIP-ergic innervation in rat atria. The period of the first few postnatal days seems to be essential for this effect, since VIP-LI concentrations in 60-day-old animals did not significantly differ between Neo-S and Neo-T atria.

Vasoactive intestinal polypeptide and calcitonin gene-related peptide in the developing rat heart atria.

Vasoactive intestinal polypeptide-like immunoreactivity (VIP-LI) and calcitonin gene-related peptide (CGRP)-LI concentrations were determined in the developing rat heart atria using radioimmunoassay. Peptide levels were analysed on postnatal days 1, 10, 25, 45, 60, and 85 (P1-P85) separately in the right (RA) and left atria (LA). No sex differences were revealed at any age examined. VIP-LI has been already detected in both atria at P1 in concentrations comparable to values at P10. In the RA, VIP-LI levels increased significantly between days P10 and P25, remained high at P45 and then declined. In the LA, VIP-LI concentrations did not differ from those in the RA on days P1, P10, P25, and P45. However, regional differences were found at P60 and P85, when the peptide levels were significantly higher in the LA than in the RA. The postnatal changes in CGRP-LI concentrations were comparable in both atria with similar values at P1 and P85. After birth, CGRP levels decreased gradually till P45, then they increased till P60 and declined again at P85. The results demonstrate that there is an asymmetry in the postnatal development of the atrial VIP-LI and CGRP-LI concentrations. VIP-LI levels reached their maximum at P25, whereas CGRP-LI levels at P60. Relatively high peptide concentrations in neonatal atria and their variations during development might be related to diverse trophic functions of VIP and CGRP.

Postnatal development of the rat intrinsic cardiac nervous system: a confocal laser scanning microscopy study in whole-mount atria.

We used confocal laser scanning microscopy and fluorescent immunohistochemistry to study the developmental pattern and distribution of specific neuronal phenotypes within the intrinsic cardiac nervous system in whole-mount atrial preparations from newborn to 5 week old rats. Individual ganglia and neuronal cell bodies were localized by means of two general neuronal markers: protein gene product 9.5 (PGP) and microtubule-associated protein two (MAP). In rats < or =2 weeks old there were two main subpopulations of intrinsic neurons located in the intraatrial septum and around the origin of the superior vena cava. The more abundant was a population of strongly tyrosine hydroxylase (TH) immunoreactive (IR) neurons (10-40 microm in diameter) most of which were also PGP-IR. The second, less numerous (approximately 60-70% than the TH-IR group) type of neurons exhibited ChAT-IR which colocalized with MAP-IR. Towards the end of the second postnatal week and during the third, the ganglia containing these neurons became more numerous and their localization also included tissues around the origins of the inferior vena cava and the pulmonary veins, as well as both atrial walls close to the AV junction. During the second and third postnatal weeks, when the extrinsic innervation of the adrenergic and cholinergic phenotypes largely increases, the intrinsic innervation also changed greatly, and around the 21st postnatal day it appeared to acquire mature characteristics. The TH-IR neurons changed their characteristics and formed two types of ganglia. The larger ganglia containing large cells (20-40 microm in diameter) expressed TH-IR mostly close to their inner body surface (approximately 80-90% of identified neurons). Most of these neurons also expressed neuropeptide Y (NPY)-IR, specifically around their nuclei. The second type of small strongly TH-IR neurons (approximately 10% of all identified neurons) were contained in smaller groups (20-50 cells) which were usually embedded into much larger ganglia (100-400 cells), containing large (20-50 microm) neurons. Unlike all other intrinsic neurons, these small TH-IR cells did not exhibit any PGP-IR or MAP-IR. The number of ChAT-IR neurons increased at this stage, reaching approximately 90% of the neurons identified by the general neuronal markers. These neurons were surrounded by a rich network of cholinergic varicose nerve fibers, some of which were likely of an extrinsic origin. We have also identified relatively small ganglia expressing immunoreactivity to vasoactive intestinal polypeptide (VIP), and to substance P (SP). The presented data indicate that the phenotypes of intrinsic neurons in the rat heart change greatly during the first month of postnatal development. This may be at least partially related to the development and maturation of functional extrinsic nervous control of the heart.

Distribution of peptide-containing neurons in the developing rat right atrium, studied using immunofluorescence and confocal laser scanning.

The developmental pattern and distribution of peptide-containing neurons in the rat heart right atrium has been studied by indirect immunofluorescence. Antibodies against neuropeptide Y (NPY), substance P (SP), and vasoactive intestinal polypeptide (VIP) were applied to whole-mount stretch preparations of the right atria from hearts of newborn to 40 day-old animals. NPY-like immunoreactivity (L1) was compared with the synaptic vesicle marker SV2 in double immunoincubation studies. The distribution of immunofluorescence was studied by confocal laser scanning microscopy. NPY-L1 and SP-L1 were present throughout the atria already at birth, in contrast to VIP-L1 that was observed at day 10. The postnatal changes of innervation were basically quantitative, with an increase in density of nerve fibres and number of varicosities, while the basic pattern of innervation was essentially established during the first 1-10 days. NPY- and SP-positive bundles of fibres appeared to enter the right atrium along the superior caval vein, having extrinsic origins. Nerve fibres with NPY-L1 colocalized in most nerve terminals with SV2-L1, and showed a developmental pattern similar to that observed for adrenergic neurons earlier. These NPY/SV2 positive fibres probably represent the extrinsic NPY innervation. In addition, NPY-L1 was identified in large intrinsic nerve cells bodies located near the atrioventricular (AV) region. Most of the VIP-L1 was observed in short nerve fibres originating in intrinsic VIP-positive cell bodies, but a few apparently extrinsic VIP-positive fibres were found, probably representing preganglionic parasympathetic neurons. SP in the atria was probably of extrinsic (sensory) origin and no nerve cell bodies with SP-L1 were detected. The results show that the peptidergic innervation in the developing rat right atrium involves both extrinsic and intrinsic peptidergic neurons which may participate in the regulation of neurotransmission in local neuronal circuits.

The postnatal development of tyrosine hydroxylase immunoreactive nerves in rat atrium, studied with immunofluorescence and confocal laser scanning microscopy.

The developmental pattern and distribution of tyrosine hydroxylase (TH) immunoreactive nerves in the rat heart has been studied by indirect immunofluorescence. Anti-TH and monoclonal antibodies against the synaptic vesicle antigen SV2 were applied to whole-mount stretch preparations of the right atria from hearts of newborn to 40-day-old animals. Immunofluorescence was studied with confocal laser scanning microscopy (CFLM). Nerve fibres with TH-like immunoreactivity (TH-LI) were present throughout the atria already at birth, with the highest density around the sino-atrial (SA) node, colocalized in most nerve terminal arborizations with the synaptic vesicle marker SV2. Also, TH-positive ganglion cells were present around the openings of the caval veins. In comparison with TH-positive fibres, SV2-positive nerve terminal fibres were more delicate, and the number of fibres larger in all areas studied. In axon bundles, TH-LI was very prominent, while the SV2-LI was weak. A gradual increase in the density of innervation was observed up to the age of 40 days. The density of innervation decreased from the epicardium to the endocardium, confirmed in cross-sections of the cardiac wall. The relative density of TH-positive nerve terminals was estimated in the CFLM from the area around the SA node. At birth, about 600 white (strongly fluorescent) pixels were registered per scanned field, with the pixel number increasing gradually up to 20,000 pixels per frame in 18- and 40-day-old animals. The results show that TH-positive, possibly adrenergic, nerve fibres are present at birth, and that the pattern is qualitatively similar to that observed in adults, with the expression of synaptic vesicle antigen being present some days before the development of noradrenaline neuroeffector maturation, as shown in a number of physiological and pharmacological studies.

[The effect increased choline levels on the synthesis and release of acetylcholine in heart atria in white rats]. / Vliv zvýsené hladiny cholinu na syntézu a uvolnování acetylcholinu v srdecních síních bílých potkanu.

The paper addresses the problem of (1) the relationship of choline in extracellular fluid and acetylcholine (ACh) synthesis in chambers of the heart of white rats, and (2) the possibility of physiological involvement of this ACh in the control of cardiac activity. Within 60 min after s.c. administration of choline in the dose of 300 mg/kg body weight the ACh content rose to 136% of the control value in isolated heart chambers and after administration of 400 mg/kg body weight to 159% of the control value. Increased ACh synthesis failed to affect the heart rate, nor did it increase the tonic effect of vagus innervation of the heart. Excessive ACh could however be released by electric transmural stimulation of isolated heart chambers as well as by potassium depolarization. The release depended on the presence of calcium ions. The obtained results demonstrate that an increase in the level of choline in extracellular fluid results in increased ACh synthesis in nerve terminals. Enhanced release of this ACh is a precondition for high choline levels to induce an increase in parasympathetic control of cardiac function.

Postnatal development of the high-affinity uptake of choline and of the synthesis of acetylcholine in rat heart atria.

Isolated heart atria from rats of different ages were incubated in a medium containing (14C)choline and the rates of the uptake of (14C)choline into the tissue and of its conversion to (14C)acetylcholine (ACh) were measured. The synthesis of (14C)ACh (expressed per 1 g of fresh weight) increased from birth until 30 days of age and diminished after 40 days of postnatal life. The rate of (14C)ACh synthesis was considerably diminished when Na+ was omitted from the incubation medium or when hemicholinium-3 was added to it; these effects of the absence of Na+ and of hemicholinium-3 were already manifest on the 1st day after birth, indicating that the sodium-dependent high-affinity uptake of choline is operative and takes part in the synthesis of ACh in the heart from the start of postnatal life (if not earlier). In newborn rats, 4% of the (14C)choline that had been taken up by the atria was converted to (14C)Ach; this proportion rose to 7-9% at the age of 20 and 30 days and in adulthood. The total uptake of (14C)choline expressed per whole atria kept increasing from birth till adulthood when related to the whole atria, but it diminished when related to 1 g of atrial weight.

Postnatal changes in the activities of acetylcholinesterase and butyrylcholinesterase in rat heart atria.

Postnatal development of the activities of acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) in rat heart atria has been investigated with the use of 1.5-bis (allyldimethylammoniumphenyl) pentane-3-dibromide (BW 284 C51) as a selective inhibitor of AChE. Total cholinesterase activity (mumol acetylthiocholine hydrolysed X g-1 per hour) increased from 218 on the 1st day after birth to 426 on the 30th day and diminished to 340 in adult rats. The activity of AChE (mumol acetylthiocholine hydrolysed X g-1 per hour) underwent more dramatic changes, increasing more than 4-fold during the first month of life, from 13 on the 1st to 58 on the 30th day of life and then decreasing to 42 in adult rats. The proportion of AChE on total cholinesterase activity increased from 6% on the 1st day to 12-15% in animals aged 24 days and more. Since AChE is known to be specifically involved in the termination of the action of acetylcholine in the sinoatrial node, the observed postnatal changes in its activity are likely to play a role in the postnatal development of cardiac parasympathetic control.

Muscarinic acetylcholine receptors in the heart of rats before and after birth.

Atropine-displaceable binding of (3H)quinuclidinyl benzilate (QNB) to homogenates was used to identify the muscarinic binding sites in rat heart atria and ventricles and to investigate developmental changes in their concentration and binding properties between the 15th day of prenatal life and 3 months after birth. On the 15th day of prenatal life, muscarinic binding sites were already present in the heart. Their concentration increased steeply between the 15th and 19th days of prenatal development; in the atria, it remained high until the 1st day after birth and thereafter it diminished throughout the postnatal life, while in the ventricles the decrease started before the first postnatal day. The concentration of the binding sites was 1.8-3.0 times higher in the atria than in the ventricles at all time points investigated. Their affinity for QNB (the antagonist) was the same in the atria and ventricles and did not change during postnatal development (KD of 17.8 pmol/l at an infinitely low concentration of the binding sites). The binding of carbamoylcholine (the agonist) to muscarinic binding sites was analysed in experiments with the displacement of (3H)QNB binding, assuming the presence of high- and low-affinity binding sites for agonists. The proportion between the concentrations of the two classes of agonist binding sites is close to 1:1 both in the atria and the ventricles and does not change with age. No statistical significant differences were discovered between the affinities of the high- and low-affinity binding sites for carbamoylcholine between the atria and the ventricles and between new-born and adult rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Choline acetyltransferase activity and distribution in rat hearts after bilateral cervical vagotomy.

The activity of choline acetyltransferase (ChAt; E.C. 2.3.1.6) measured as the bromoacetylcholine-sensitive portion of the maximal rate of acetylcholine synthesis has been determined in homogenates of nine regions of the heart of control rats and rats sacrificed 72-74 h after bilateral cervical vagotomy. When related to the content of protein, the activity of ChAT was lowered by 30% in the atria and unchanged in the ventricles of vagotomized rats. The highest absolute decline caused by vagotomy was observed in the sinoatrial region; it was somewhat less in the rest of the right atrium and in the interatrial septum and considerably less in the left atrium. It is concluded that preganglionic parasympathetic fibres supply mainly the sinoatrial region and the right atrium, and that they do not branch to the ventricles. Preganglionic ChAT nts about 30% of total ChAT activity in the atria. The sinoatrio-ventricular gradient in the distribution of ChAT in the heart is due to uneven distribution of both the pre- and postganglionic ChAT pools (i.e., of both the pre- and postganglionic cholinergic nerve fibres and nerve cell bodies).