Centrally regulated blood pressure response to vasoactive peptides is modulated by corticosterone.

To investigate the role of brain glucocorticoid (GR) and mineralocorticoid receptors (MR) in centrally evoked blood pressure responses, the effects of intracerebroventricular (i.c.v.) administration of angiotensin II and vasopressin were studied in adrenalectomized rats with and without corticosterone or aldosterone replacement. Five groups were examined: (i) Adrenalectomy (ADX); (ii) ADX + a subcutaneously implanted 20-mg corticosterone pellet (low corticosterone); (iii) ADX + 100 mg corticosterone pellet (high corticosterone); (iv) ADX + 6 microg/24 h aldosterone via Alzet minipump (Aldo); and (v) Sham adrenalectomy (Sham). Pressor responses to 150 ng angiotensin II and 50 ng vasopressin i.c.v. were determined in freely moving rats using biotelemetry. The results show that, compared to sham rats, ADX rats showed significantly reduced pressor responses. This reduction of the pressor response to angiotensin II could be reversed and even further enhanced by replacement of the ADX rats with high corticosterone concentrations. In contrast, with aldosterone, a depressor type response was observed. Corticosterone replacement could not restore the pressor response to vasopressin. We conclude that the pressor response to centrally administered vasoactive substances is substantially attenuated by removal of the adrenals and that, in the case of angiotensin II, this is due to the lack of high concentrations of circulating corticosterone occupying both MR and GR. However, predominant MR occupancy appears to play an opposite role and attenuates the angiotensin II-induced pressor response.

Intracerebroventricular administration of a glucocorticoid receptor antagonist enhances the cardiovascular responses to brief restraint stress.

Intracerebroventricular (i.c.v.) administration of the glucocorticoid receptor antagonist 17beta-hydroxy-11beta(4-dimethylamino-phenyl)17alpha-(1-propynyl)estra-4,9dien-3one (RU38486) in conscious rats slowly increased systolic blood pressure as assessed with the indirect tail cuff method. However, direct measurement of blood pressure in freely moving rats did not reveal changes in blood pressure after i.c.v. injection of this antagonist either in the light or in the dark phase. In the present study, the hypothesis is tested that aspects of the tail cuff procedure, involving heat (30 min, 32 degrees C) and brief restraint stress, are necessary conditions to detect the glucocorticoid receptor-mediated cardiovascular effect. Freely moving rats equipped with a telemetric transmitter to directly measure heart rate and blood pressure were injected i.c.v. with either the glucocorticoid receptor or the mineralocorticoid receptor antagonist and were either left undisturbed for 24 h, or were subjected to the tail cuff procedure at 1.5, 6.5 and 23.5 h after injection. Then after 30-min warming and during brief restraint, blood pressure and heart rate showed a rapid increase. The mineralocorticoid receptor antagonist administered i.c.v. did not affect these stress-induced increases in cardiovascular responses. The glucocorticoid receptor antagonist i.c.v. significantly increased the heart rate and pressor response at 24 h. In the undisturbed rats, neither basal heart rate nor blood pressure were affected by either antagonist during the circadian cycle. In conclusion, the blockade of central glucocorticoid receptor causes a long-lasting facilitation of the stress-induced pressor and heart rate response, which does not require a 2-week training to the condition of heat and stress.

Blood pressure, heart rate, and behavioral responses to psychological "novelty" stress in freely moving rats.

We developed a new model of psychological "open-field" stress in freely moving rats. Blood pressure and heart rate of the rats were measured by radiotelemetry and behavior analyzed by video tracking software. Open-field exposure induced marked increases in blood pressure and heart rate. Repeated daily exposure induced pressor responses that were slightly higher on Day 4 when compared to Day 1. Pretreatment with the beta1-adrenoceptor antagonist atenolol inhibited the tachycardia whereas the ganglion blocker pentolinium inhibited the pressor response, indicating involvement of the sympathetic nervous system. Pretreatment with diazepam prevented the novelty stress-induced pressor response and reduced the tachycardia. These results show that the psychological stress of exposing rats to an open field induces marked cardiovascular effects that are mediated by sympathetic hyperactivity. This model is unique in that it focuses on psychological stress and allows concomitant measurement of blood pressure, heart rate, and behavior in freely moving rats.

7-monohydroxyethylrutoside protects against chronic doxorubicin-induced cardiotoxicity when administered only once per week.

Doxorubicin is a very effective antitumor agent, but its clinical use is limited by the occurrence of a cumulative dose-related cardiotoxicity, resulting in congestive heart failure. 7-Monohydroxyethylrutoside (monoHER), a flavonoid belonging to the semisynthetic hydroxyethylrutoside family, has been shown to protect against doxorubicin-induced cardiotoxicity when administered i.p. at a dose of 500 mg/kg five times/week in combination with a weekly i.v. dose of doxorubicin. Such a dosing schedule would be very inconvenient in clinical practice. We therefore investigated a dosing schedule of one administration of monoHER just before doxorubicin. The electrocardiogram was measured telemetrically in mice after the combined treatment of doxorubicin (4 mg/kg, i.v.) with one dose of monoHER (500 mg/kg, i.p., administered 1 h before doxorubicin) for 6 weeks. These data were compared with the five times/week schedule (500 mg/kg, i.p., administered 1 h before doxorubicin and every 24 h for 4 days). The increase of the ST interval was used as a measure for cardiotoxicity. It was shown that 500 mg/kg monoHER administered only 1 h before doxorubicin provided complete protection against the cardiotoxicity. This protection was present for at least 10 weeks after the last treatment. Because of the short half-life of monoHER, these results suggest that the presence of monoHER is only necessary during the highest plasma levels of doxorubicin.

Brain mineralocorticoid receptors and centrally regulated functions.

Mineralocorticoid receptors (MRs) expressed in limbic neurons, notably of hippocampus, retain both aldosterone and corticosterone. Basal concentrations of corticosterone already substantially occupy the limbic MR type, suggesting that in hippocampal neurons, MR activity rather than ligand bioavailability is rate limiting. The periventricular region expresses MRs involved in the control of salt homeostasis, which are aldosterone selective because of the presence of 11beta-hydroxysteroid dehydrogenase. MR is in hippocampal CA1, CA2, and dentate gyrus colocalized with glucocorticoid receptors (GRs). Both receptor types mediate in a coordinate manner the corticosterone action on information processing critical for behavioral adaptation and associated neuroendocrine responses to stress. MRs operate in proactive mode determining the sensitivity of the stress response system, while GRs facilitate recovery from stress in reactive mode. On the neuronal level, MR-mediated action maintains a stable excitatory tone and attenuates the influence of modulatory signals. In contrast, GR-mediated effects suppress excitability transiently raised by excitatory stimuli. MR is also involved in control of autonomic outflow and volume regulation. This was demonstrated by the effect of an MR antagonist, which was administered centrally, because mdr P-glycoproteins hamper the access of synthetic steroids to the brain. The MR antagonist attenuates pressor responses to a stressor, such as experienced during tail sphygmography. Diuresis and urinary electrolyte excretion are increased after the MR antagonist, but this effect is abolished after bilateral denervation of the kidney. It is presently unknown in which brain cells the MR-mediated effects on these aspects of central cardiovascular regulation occur.

In vitro screening of antitumour agents for cardiotoxicity by means of isolated mouse left atria.

Cardiotoxicity, a side-effect that can occur after treatment with an anticancer drug, has severe clinical implications. Therefore, a model is desired to screen new anticancer drugs or drug combinations for possible cardiotoxic side-effects. In the present study we tested the applicability of the electrically stimulated isolated mouse left atrium model using a wide range of anticancer drugs with known cardiotoxicity. It appeared that the cardiotoxicity observed in our model, i.e. the negative or positive inotropic effects of the drugs on the isolated atrium, corresponded with the observed cardiotoxicity in animals and/or humans. It is therefore concluded that our model can be used to wam for possible cardiotoxic side-effects of anticancer drugs in vivo.

Influence of iron chelation on the antioxidant activity of flavonoids.

The antioxidant activity of flavonoids is believed to be caused by a combination of iron chelation and free radical scavenging activities. Several authors have attempted to separate the iron chelation and scavenging activity of flavonoids in order to study these processes individually. There are, however, several contradictions in the literature, and the outcome largely depends on the experimental conditions and the type of assay used. In order to investigate the contribution of iron chelation to the antioxidant activity of flavonoids, we determined the antioxidant activity of a group of flavonoids from several subclasses in an iron-independent (azobisamidinopropane, [ABAP]) lipid peroxidation (LPO) process and compared them with data from an iron-dependent (Fe2+/ascorbate) LPO process, which we determined earlier. These LPO data were compared with oxidation potentials, which were earlier found to have a good correlation with the scavenging activity of flavonoids. For most flavonoids, it was found that there was no difference between the LPO assays, indicating that iron chelation is either a constant factor among the flavonoids or is not significant in these types of assays. The IC50 values in the iron-independent LPO assay showed an excellent correlation with the oxidation potentials (Ep/2). Therefore, it can be concluded that for the majority of flavonoids tested, iron chelation does not play a role in the antioxidant activity in microsomal lipid peroxidation.

Monohydroxyethylrutoside, a dose-dependent cardioprotective agent, does not affect the antitumor activity of doxorubicin.

The cumulative dose-related cardiotoxicity of doxorubicin is believed to be caused by the production of oxygen- free radicals. 7-Monohydroxyethylrutoside (monoHER), a semisynthetic flavonoid and powerful antioxidant, was investigated with respect to the prevention of doxorubicin-induced cardiotoxicity in mice and to its influence on the antitumor activity of doxorubicin in vitro and in vivo. Non-tumor-bearing mice were equipped with a telemeter in the peritoneal cavity. They were given six weekly doses of 4 mg/kg doxorubicin i.v., alone or in combination with either 100 or 250 mg/kg monoHER i.p., 1 h prior to doxorubicin administration and for the following 4 days. Cardiotoxic effects were measured from electrocardiogram changes up to 2 weeks after treatment. Protection against cardiotoxicity was found to be dose dependent, with 53 and 75% protection, respectively, as calculated from the reduction in the increase in the ST interval. MonoHER and several other flavonoids with good antioxidant properties were tested for their antiproliferative effects in the absence or the presence of doxorubicin in A2780 and OVCAR-3 human ovarian cancer cells and MCF-7 human breast cancer cells in vitro. Some flavonoids were directly toxic at 50 and 100 microM, whereas others, including monoHER, did not influence the antiproliferative effects of doxorubicin at these concentrations. The influence of monoHER was further tested on the growth-inhibitory effect of 8 mg/kg doxorubicin i.v., given twice with an interval of 1 week in A2780 and OVCAR-3 cells that were grown as s.c. xenografts in nude mice. MonoHER, administered 1 h before doxorubicin in a dose schedule of 500 mg/kg i.p. 2 or 5 days per week, was not toxic and did not decrease the antitumor activity of doxorubicin. It can be concluded that monoHER showed a dose-dependent protection against chronic cardiotoxicity and did not influence the antitumor activity of doxorubicin in vitro or in vivo.

A quantum chemical explanation of the antioxidant activity of flavonoids.

Flavonoids are a group of naturally occurring antioxidants, which over the past years have gained tremendous interest because of their possible therapeutic applicability. The mechanism of their antioxidant activity has been extensively studied over several decades. However, there is still much confusion about the molecular mechanism of radical scavenging and the relationship between structure and activity. Therefore, we have calculated the heat of formation and the geometry of both the parent compound and the corresponding radical using the ab initio program GAMESS. We have compared their differences in energy in order to gain insight into the stability of the radical and the ease with which it is formed. We have also investigated the spin density of the radical, to determine the delocalization possibilities. These calculated data were compared with experimental data from ESR (hyperfine coupling constants) and electrochemical oxidation (Ep/2) and were found to be in good agreement. By comparing the geometries of several flavonoids, we were able to explain the structural dependency of the antioxidant action of these compounds. The extremely good antioxidant activity of the flavonols could be explained by the formation of an intramolecular hydrogen bond.

Structural aspects of antioxidant activity of flavonoids.

Flavonoids, a group of naturally occurring antioxidants and iron chelators, might be used as cardioprotective agents in doxorubicin-induced cardiotoxicity, which is believed to be caused by the formation of oxygen free radicals. To investigate the underlying molecular mechanism, we tested a large group of flavonoids from all major structural subclasses on their ability to inhibit doxorubicin (enzymatically)-induced and Fe2+/ascorbate (nonenzymatically)-induced microsomal lipid peroxidation (LPO) and to chelate Fe2+. In addition, we measured half peak oxidation potentials (Ep/2). LPO inhibition data gave a good qualitative correlation with the oxidation potentials. Most flavonoids tested chelated Fe2+, but there were large differences in the chelating capacity. For good scavenging activity, a catechol moiety on ring B is required. The 3-OH moiety can function as a chelation site and can also be oxidized. The 3-OH group in combination with a C2 C3 double bond, increases the scavenging activity. Fe2+ chelation only plays a role in the LPO inhibition by less active scavengers. Chelation can then raise the activity to the level of the most active scavengers, possibly by site-specific scavenging. It can be concluded that Ep/2 values and iron chelating activity can almost completely describe the LPO inhibiting behaviour of the flavonoids.

Doxorubicin-induced cardiotoxicity monitored by ECG in freely moving mice. A new model to test potential protectors.

In laboratory animals, histology is most commonly used to study doxorubicin-induced cardiotoxicity. However, for monitoring during treatment, large numbers of animals are needed. Recently we developed a new method to measure ECG values in freely moving mice by telemetry. With this model we investigated the effect of chronic doxorubicin administration on the ECG of freely moving BALB/c mice and the efficacy of ICRF-187 as a protective agent. The ST interval significantly widened from 15.0 +/- 1.5 to 56.8 +/- 11.8 ms in week 10 (7 weekly doses of 4 mg/kg doxorubicin given i.v. plus 3 weeks of observation). The ECG of the control animals did not change during the entire study. After sacrifice the hearts of doxorubicin-treated animals were enlarged and the atria were hypertrophic. As this schedule exerted more toxicity than needed to investigate protective agents, the protection of ICRF-187 was determined using a dose schedule with lower general toxicity (6 weekly doses of 4 mg/kg doxorubicin given i.v. plus 2 weeks of observation). On this schedule, the animals' hearts appeared normal after sacrifice and ICRF-187 (50 mg/kg given i.p. 1 h before doxorubicin) provided almost full protection. These data were confirmed by histology. The results indicate that this new model is very sensitive and enables monitoring of the development of cardiotoxicity with time. These findings result in a model that allows the testing of protectors against doxorubicin-induced cardiotoxicity as demonstrated by the protection provided by ICRF-187.

Effect of dimethyl sulfoxide (DMSO) on the electrocardiogram (ECG) in freely moving male Balb/c mice.

1. Since dimethyl sulfoxide (DMSO) is a solvent which is often used for drugs in animal studies, we investigated the effect of a daily administration of DMSO on the telemetrically obtained electrocardiogram (ECG) in freely moving male Balb/c mice. 2. During treatment with 4.5 ml 100% DMSO/kg i.p. 5 days per week during 3 weeks, DMSO caused substantial cardiotoxicity. The ST-interval increased significantly after 1 week by 2.2 +/- 1.3 msec and also the ECG wave form changed completely in time. 3. During treatment with 4.5 ml 50% DMSO/kg i.p. 5 days per week during 3 weeks, no significant difference was observed compared with the control animals. 4. During the entire study the maximal heart rate and body weight remained constant in all treated groups. 5. The data indicate that DMSO can not be used in a 100% concentration to dissolve compounds that are tested for protection against the cardiotoxicity of cytostatics.

Flavonoids as scavengers of nitric oxide radical.

Flavonoids are a group of naturally occurring compounds used, e.g., in the treatment of vascular endothelial damage. They are known to be excellent scavengers of oxygen free radicals. Since the nitric oxide radical (.NO) probably plays a role in this pathology, the .NO scavenging capacity of the flavonoids was determined. It was found that the flavonoids are very potent .NO scavengers. The anthocyanidins were found to be more effective scavengers than the hydroxyethylrutosides, which correlated with their therapeutic activity. The values of their scavenging rate constants are only 30 times less active than the very potent endogenous .NO scavenger haemoglobin. It is speculated that .NO scavenging plays a role in the therapeutic effect of the flavonoids.

Monohydroxyethylrutoside as protector against chronic doxorubicin-induced cardiotoxicity.

1. The clinical use of the antitumour agent, doxorubicin, is largely limited by the development of a cumulative dose-related cardiotoxicity. This toxicity is generally believed to be caused by the formation of oxygen free radicals. In earlier studies it was established that flavonoids, naturally occurring antioxidants, can provide some degree of protection. In this study we investigated whether 7-monohydroxyethylrutoside (monoHER), a powerful antioxidative flavonoid with extremely low toxicity, can provide protection to an extent comparable to the clinically successful Cardioxane (ICRF-187). 2. Balb/c mice of 20-25 g were equipped i.p. with a telemeter to measure ECG. They were given 6 i.v. doses of doxorubicin (4 mg kg-1) at weekly intervals. ICRF-187 (50 mg kg-1) or monoHER (500 mg kg-1) were administered i.p. 1 h before doxorubicin administration. In the 2 monoHER groups the treatment continued with either 1 or 4 additional injections per week. A saline and monoHER treated group served as controls. After these 6 weeks, they were observed for another 2 weeks. 3. At the end of this study (week 8) the ST interval had increased by 16.7 +/- 2.7 ms (mean +/- s.e. mean) in doxorubicin-treated mice. At that time, the ST interval had increased by only 1.8 +/- 0.9 ms in ICRF-187 co-mediated mice and in monoHER co-medicated mice by only 1.7 +/- 0.8 and 5.1 +/- 1.7 ms (5- and 2-day schedule, respectively, all P < 0.001 relative to doxorubicin and not significantly different from control). The ECG of the control animals did not change during the entire study. The QRS complex did not change in either group.4. It can be concluded that monoHER protects against doxorubicin-induced cardiotoxicity and merits further evaluation in this respect.

Comparison of different iron chelators as protective agents against acute doxorubicin-induced cardiotoxicity.

Doxorubicin (Dox) is a widely used antineoplastic agent. Irreversible cardiomyopathy is a serious and dose-limiting side effect after chronic administration. The iron chelating bispiperazinedione ICRF-187 is currently the only drug which affords protection against Dox-induced cardiotoxicity. To compare the protective value of structurally unrelated iron chelators, isolated mice atria were exposed to Dox (30 microM) and either the hydroxamate desferrioxamine (DFO, 200 and 500 microM), EDTA (200 microM) or the hydroxypridones CP44 (200 microM), CP51 (200 microM), and CP93 (200 microM) and ICRF-187 (200 and 500 microM). The nitroxide TEMPO (5 mM) lacks iron chelating properties but was used to prevent redox cycling or iron and scavenge superoxide. All iron chelators, except EDTA. CP93 and CP44, were modestly protective against a Dox-induced decrease in contractile force. As a single agent the hydroxypridones decreased atrial contractile force. At a concentration of 200 microM, DFO was the most effective protector of the chelators tested. However, this effect disappeared when a concentration of 500 microM was used. This in contrast to ICRF-187 for which a concentration-dependent inhibition of Dox-induced decrease in contractile force was observed. TEMPO exerted a biphasic response consisting of a two-fold increase in contractile force, followed by a decrease in force and irregular contractions. In this model TEMPO lacked any perspective as a cardioprotectant. We conclude that at 200 microM. DFO was the most effective agent to afford protection against Dox-mediated atrial malfunction. However, at 500 microM, DFO was not effective whereas ICRF-187 afforded partial protection. Hydroxipyridones were found to be of limited value because of a negative inotropic effect on the isolated atria.

Use of telemetry to record electrocardiogram and heart rate in freely moving mice.

This paper describes for the first time the possibility to record the electrocardiogram (ECG) and heart rate (HR) with a commercially available telemetry and data acquisition system in freely moving mice. The system comprises a telemetry transmitter implanted in the peritoneal cavity and a receiver, placed underneath the home cage, an A/D converter (MacLab) and a Macintosh LC II 4/80 computer with software (MacLab, Chart/Scope). The raw analog ECG data are digitized within the MacLab and can be converted to HR data additionally. The effects of surgery for implanting the transmitter, handling and anesthesia by either Nembutal or a mixture of Hypnorm, Dormicum, and water, on the changes in ECG and HR were examined. The telemetry system for recording the ECG and HR provides an accurate and reliable method for monitoring the direct effects of handling on HR. By using this telemetry system, we maintain that measurements in freely moving animals are more efficient, reliable, and less labor-intensive than the measurement techniques described in the literature thus far.

Cardioprotective properties of O-(beta-hydroxyethyl)-rutosides in doxorubicin-pretreated BALB/c mice.

Chronic doxorubicin-induced cardiotoxicity is believed to be caused by the formation of oxygen free radicals. Thus O-(beta-hydroxyethyl)-rutosides, a standardized flavonoid mixture (Venoruton) with iron chelating and radical scavenging activity, might provide protection. Therefore, we investigated the (cardio)protective effect of Venoruton (1.5 g/kg injected i.p. on days 1-5, 8-12, 15-19, and 22-26) in BALB/c mice treated with doxorubicin (4 mg/kg injected i.v. on days 1, 8, 15, and 22) compared with mice treated with doxorubicin alone. Saline-treated animals served as controls. No mortality was encountered in either of the groups; weight gain data suggest little general toxicity of this dose schedule. The basal frequency of the isolated right atria was increased in doxorubicin-pretreated animals as compared to control animals (468 +/- 22 and 366 +/- 20 beats/min, respectively). Venoruton coadministration diminished this increase (373 +/- 17 beats/min). The -log of the concentration giving 50% effect of l-isoprenaline on the right atrium was changed after doxorubicin pretreatment (8.33 +/- 0.04 versus 8.86 +/- 0.06 for control animals). Venoruton coadministration resulted in a smaller shift in the -log of the concentration giving 50% effect (8.51 +/- 0.10) than with doxorubicin alone. The extent of cardiotoxicity found in the functional studies was confirmed by histological scoring of heart ventricle damage. It can be concluded that Venoruton has the potential to protect against doxorubicin-induced cardiotoxicity.

Molecular pharmacology of vitamin E: structural aspects of antioxidant activity.

In this review, the involvement of vitamin E in free radical physiology and antioxidant mechanisms is discussed. Moreover, structure-activity relationship (SAR) studies on vitamin E analogues are presented. A molecular explanation for the antioxidant activity often is based on molecular parameters, such as Hammett sigma and Brown sigma +. These parameters correlate with the activity. Using semiempirical calculations, we have found other molecular parameters related to electron distribution and structure (such as the difference in heat of formation between the compound and its radical or the energy of the highest occupied molecular orbital, HOMO) which correlate with the antioxidant action of vitamin E and its derivatives.

A predictive model for substrates of cytochrome P450-debrisoquine (2D6).

Molecular modeling techniques were used to derive a predictive model for substrates of cytochrome P450 2D6, an isozyme known to metabolize only compounds with one or more basic nitrogen atoms. Sixteen substrates, accounting for 23 metabolic reactions, with a distance of either 5 A ("5-A substrates", e.g., debrisoquine) or 7 A ("7-A substrates", e.g., dextromethorphan) between oxidation site and basic nitrogen atom were fitted into one model by postulating an interaction of the basic nitrogen atom with a negatively charged carboxylate group on the protein. This acidic residue anchors and neutralizes the positively charged basic nitrogen atom of the substrates. In case of "5-A substrates" this interaction probably occurs with the carboxylic oxygen atom nearest to the oxidation site, whereas in the case of "7-A substrates" this interaction takes place at the other oxygen atom. Furthermore, all substrates exhibit a coplanar conformation near the oxidation site and have negative molecular electrostatic potentials (MEPs) in a part of this planar domain approximately 3 A away from the oxidation site. No common features were found in the neighbourhood of the basic nitrogen atom of the substrates studied so that this region of the active site can accommodate a variety of N-substituents. Therefore, the substrate specificity of P450 2D6 most likely is determined by the distance between oxidation site and basic nitrogen atom, by steric constraints near the oxidation site, and by the degree of complementarity between the MEPs of substrate and protein in the planar region adjacent to the oxidation site.(ABSTRACT TRUNCATED AT 250 WORDS)

4- or 5- (omega-aminoalkyl) thiazoles and derivatives; new selective H2-receptor agonists.

It is well known that both histamine and dimaprit show moderate H2-receptor agonistic activity (guinea pig right atrium). Quantum chemical calculations indicated that 2-aminothiazole derivatives that might be regarded as cyclic dimaprit analogues, should possess H2-receptor agonistic activity as well. In the present study a series of 4- or 5-(omega-aminoalkyl) thiazoles has been synthesized, showing a moderate to strong H2-receptor agonistic activity as compared to histamine whereas no activity on H1- and H3-receptors could be detected. In contrast to histamine and derivatives, which are supposed to "trigger" the H2-receptor via a tautomeric shift involving two protons viz. one proton of the active site of the receptor and one proton of the heteroaromatic ring system, the thiazole derivatives seem to stimulate the H2-receptor via a one proton mechanism. In a series of impromidine analogues the 3- [4(5)-imidazolyl]propyl moiety was replaced by the more H2-receptor specific 3-(4- or 5-thiazolyl)propyl fragment resulting in potent and selective full H2-receptor agonists.