Peptides.Vol. 11, pp. 939-944. ©Pergamon Press plc, 1990. Printed in the U.S.A.

0196-9781/90 $3.00 + .00

Selective Cardiorespiratory Activity of an Iodinated Analog of Thyrotropin-Releasing Hormone (TRH) I L A R I P A A K K A R I , * ? l A S K O J A R V I N E N , t S T E F A N V O N H O F , $ P E K K A T. M , ~ N N I S T O , ? L O U I S A. C O H E N , : ~ V I R E N D E R M . L A B R O O : ~ A N D G I O R A F E U E R S T E I N *

*Department of Neurology, Uniformed Services University of the Health Sciences, Bethesda, MD 20814-4799 "kDepartment of Pharmacology and Toxicology, University of Helsinki Siltavuorenpenger 10, SF-00170 Helsinki, Finland -7:Laboratory of Chemistry, NIDDK, National Institutes of Health, Bethesda, MD 20892 R e c e i v e d 20 F e b r u a r y 1990

PAAKKARI, I., A. L~RVINEN, S. VONHOF, P. T. MANNISTO, L. A. COHEN, V. M. LABROO AND G. FEUERSTEIN. Selective cardiorespiratory activity of an iodinated analog of thyrotropin-releasing hormone (TRH). PEPTIDES 11(5) 939-944, 1990.--The biological activity of thyrotropin-releasing hormone (TRH) and its analogs 4(5)-I-Im-TRH and 2,4(5)-I2-Im-TRH was assessed by means of their effects on: 1) the mean arterial pressure (MAP), 2) heart rate (HR), 3) ventilation minute volume (MV), 4) contractility of the rat duodenum, and 5) concentrations of thyrotropin (TSH) or prolactin (PRL) in serum. Also their binding to TRH-receptors in brain homogenates was studied. In urethane-anesthetized rats TRH ICV increased MAP, HR and MV. 4(5)-I-Im-TRH was equally as active as TRH on HR and MV but a significant elevation in MAP was observed only at a dose 100-fold to that of TRH. However, the maximal responses of 4(5)-I-Im-TRH and TRH did not differ. In conscious rats, TRH IA elevated MAP and HR but 4(5)-I-Im-TRH was active on MAP only. 2,4(5)-I2-Im-TRH was devoid of cardiorespiratory activity. TRH dose-dependently inhibited the contractions of the rat duodenum while the iodinated analogs lacked such an activity. To induce a significant release of TSH several hundred times more of 4(5)-I-Im-TRH and over 1000 times more of 2,4(5)-I2-Im-TRH were needed as compared to TRH. The iodoanalogs elevated PRL levels only at doses 2000-fold higher than those of TRH. The iodoanalogs displaced [3H][3-Me-His2]TRH ([3H]MeTRH) from its binding sites at concentrations about 1000 times higher than those of TRH. Substitutions of the histidyl moiety of TRH in 4(5)-I-Im-TRH and 2,4(5)-I2-Im-TRH resulted in substantial loss of the endocrine activity. While the di-iodinated analog was practically devoid of any biological activity the monoiodinated analog exerted similar cardiorespiratory activity to that of TRH. Blood pressure Heart rate Minute ventilation volume Prolactin Thyrotropin Thyrotropin-releasing hormone (TRH) TRH analog

EXOGENOUSLY administered thyrotropin-releasing hormone (TRH) not only releases thyrotropin (TSH) and prolactin (PRL) but it also elevates the mean arterial pressure (MAP), stimulates the heart rate (HR) and the ventilation, and exerts other central effects [for review, see (6, 21, 22)]. Several analogs have been synthesized in search of compounds that would selectively exert one or some of the effects of TRH. For example, the introduction of fluorine [4(5)-F-Im-TRH] into the imidazole ring of TRH results in prolonged cardiovascular and PRL-releasing activity (4,8). Analogs with a large electron-withdrawing substitution in the imidazole ring of the histidine moiety [4(5)-CF3-Im-TRH or 2-CF 3Im-TRH] show cardiovascular activity equal to that of TRH but exert increased PRL-releasing activity as compared to TRH (4,20). However, another analog with an electron-withdrawing substituent (4-NO2-Im-TRH) showed no change from TRH in the

Receptor binding

Smooth muscle

cardiovascular properties, but had a decreased PRL-releasing capacity (4,20). To evaluate further the structure activity relations of the histidine moiety of TRH, we have studied analogs with iodine substitution in the histidine ring. Iodine has about one-half of the electron-withdrawing capacity of the NO 2 group and it is slightly less bulky than the trifluoromethyl group. Cardiorespiratory, endocrine and smooth muscle effects of the iodoanalogs have been studied. Additionally, binding to central TRH receptors of the analogs was tested. METHOD

Cardiovascular Experiments With Anesthetized Rats Male Wistar rats (200-330 g) were kept in 12-hour light/dark

~Requests for reprints should be addressed to Ilari Paakkari, Department of Pharmacology and Toxicology, University of Helsinki, Siltavuorenpenger 10, SF-00170 Helsinki, Finland.

939

PAAKKARI ET AL.

940

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FIG. 1. Structure formulas for TRH, 4(5)-I-Im-TRH and 2,4(5)-IeIm-TRH.

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Male Sprague-Dawley rats (250-330 g) were anesthetized with

TABLE 1 M A X I M A L CHANGES IN MAP, HR A N D MV IN URETHANE-ANESTHETIZED RATS AFTER ICV INJECTIONS OF THE DRUGS

Treatment

MAP (mmHg)

NaC1 NaI TRH 4(5)-I-Im-TRH 2,4(5)-I2-Im-TRH

4 I 37 30 5

_+ 7 +- 4 -+- 7* +- 6* -+ 8

HR (bpm) 14 23 31 35 34

+ +--+ +-+

6 13 4* 7* 15

MV (ml/min) -7 6 42 5l 19

+_ +-+ ++

8 6 10* 6? 9*

The maximal changes were observed with the highest dose of 1000 nmol/kg of TRH and both analogs• Number of animals was 6-7 in each group. Mean -+- SEM are shown. *p