Pharmacological profile of the novel mammalian tachykinin, hemokinin 1

Abstract

1. The effects of the novel mammalian tachykinin, hemokinin 1 (HEK-1), have been investigated by radioligand binding and functional in vitro and in vivo experiments. 2. Similar to SP (K(i)=0.13 nM), HEK-1 inhibited in a concentration-dependent manner and with high affinity [(3)H]-substance P (SP) binding to human NK(1) receptor (K(i)=0.175 nM) while its affinity for [(125)I]-neurokinin A (NKA) binding at human NK(2) receptor was markedly lower (K(i)=560 nM). 3. In isolated bioassays HEK-1 was a full agonist at tachykinin NK(1), NK(2) and NK(3) receptors. In the rat urinary bladder (RUB) HEK-1 was about 3 fold less potent than SP. In the rabbit pulmonary artery (RPA) HEK-1 and in the guinea-pig ileum (GPI), HEK-1 was about 500 fold less potent than NKA and NKB, respectively. 4. The responses to HEK-1 were antagonized by GR 82334 in RUB (pK(B)=5.6+/-0.07), by nepadutant in RPA (pK(B)=8.6+/-0.04) and by SR 142801 in GPI (pK(B)=9.0+/-0.2) with apparent affinities comparable to that measured against tachykinin NK(1), NK(2) and NK(3) receptor-selective agonists, respectively. 5. Intravenous HEK-1 produced dose-related decrease of blood pressure in anaesthetized guinea-pigs (ED(50)=0.1 nmol kg(-1)) and salivary secretion in anaesthetized rats (ED(50)=6 nmol kg(-1)) with potencies similar to that of SP. All these effects were blocked by the selective tachykinin NK(1) receptor antagonist, SR 140333. 6. We conclude that HEK-1 is a full agonist at tachykinin NK(1), NK(2) and NK(3) receptors, possesses a remarkable selectivity for NK(1) as compared to NK(2) or NK(3) receptors and acts in vivo experiments with potency similar to that of SP.

Figure 1

Competition binding curves of unlabelled SP, NKA, septide and HEK-1 at the NK₁ receptor expressed in CHO cells membranes. 0.1 nM [³H]-SP (A) or [¹²⁵I]-NKA (B) were used as radioligand. Data shown represent the mean±s.e.mean of 3 – 4 experiments, each one performed in duplicate.

Figure 2

Concentration-response curves of the contractile effect of HEK-1 in rat urinary bladder (RUB, panel A), rabbit pulmonary artery (RPA, panel B), guinea-pig ileum (GPI, panel C) under appropriate experimental conditions for recording the tachykinin NK₁, NK₂ and NK₃ receptor-mediated responses, respectively. The experiments in the RUB were performed in the presence of SR 48968 (1 μM) and indomethacin (10 μM). The experiments in GPI were conducted in the presence of the tachykinin NK₁ receptor antagonist SR 140333 (1 μM). Concentration-response curves to HEK-1 are shown either in the absence or in the presence of GR 82334 (10 μM), nepadutant (10 nM) and SR 142801 (0.1 μM) in the RUB, RPA and GPI, respectively. Each value is the mean±s.e.mean of 4 – 7 experiments.

Figure 3

Dose-dependent (0.01 – 1000 nmol kg⁻¹) hypotensive (panels A and B) and bradycardic (panel C) effect of HEK-1 and SP in anaesthetized guinea-pigs. The maximal effect on systolic and diastolic blood pressure was reached at 10 nmol kg⁻¹ of either agonist. ^*P<0.05 significantly different from the response induced by SP. Each value is the mean±s.e.mean of 10 experiments.

Figure 4

Time-courses of the cardiovascular effects of intravenous administered SP and HEK-1 (0.01 – 10 nmol kg⁻¹) on systolic (A,B), diastolic (C,D) and heart rate (E,F) in anaesthetized guinea-pigs. The maximal response was reached within 1 – 3 min from administration of each peptide. Each value is the mean±s.e.mean of 10 experiments and represents the difference between the peak effect and the basal value at each dose.

Figure 5

Salivary secretion produced by SP and HEK-1 (0.1 – 100 nmol kg⁻¹ i.v.) in atropine-pretreated (1.4 μmol kg⁻¹ i.v., followed by infusion of 1.4 μmol ml⁻¹ in a volume of 300 μl h⁻¹) anaesthetized rats. The saliva was collected for a period of 5 min after administration of the peptide. Each value is the mean±s.e.mean of 10 experiments.

Figure 6

Effect of the tachykinin NK₁ receptor antagonist SR 140333 (1 μmol kg⁻¹ i.v., 5 min before agonist) on SP and HEK-1 (1 nmol kg⁻¹ i.v.)-induced inhibition of systolic (A), diastolic (B) blood pressure and heart rate (C) in anaesthetized guinea-pigs and on SP and HEK-1 (10 nmol kg⁻¹ i.v.)-induced salivary secretion (D) in anaesthetized rats. Empty columns for SP and full columns for HEK-1 represent the control responses in vehicle-treated animals. Each value is the mean±s.e.mean of four experiments. ^*P<0.05 significantly different from the control response.