CHARACTERIZATION OF ENDOTHELIN RECEPTORS MEDIATING THE EFFECTS OF THE ENDOTHELIN SARAFOTOXIN PEPTIDES ON AUTONOMIC NEUROTRANSMISSION IN THE RAT VAS-DEFERENS AND GUINEA-PIG ILEUM

1 To characterize the receptors mediating the effects of the endothelin/sarafotoxin family of peptides on the responses to electrical stimulation of the rat vas deferens (RVD) and guinea-pig ileum (GPI) we have used endothelin-1 (ET-1), ET-3, sarafotoxin 6b (SX6b) and SX6c as agonists and the endothelin-receptor antagonists BQ-123 (ET(A) receptor selective) and PD 142893 (non-selective). 2 In the RVD, ET-1 and SX6b increased the twitches induced by field stimulation starting at a threshold concentration of 10(-10)M while the threshold concentration for ET-3 was 3 x 10(-9)M. SX6c (up to 3 x 10(-8)M) did not potentiate the twitches. SX6b produced significantly (P < 0.05) greater potentiations than ET-1 at concentrations of 3 x 10(-9)M and higher, and 10(-7)M ET-3 also produced a significantly greater effect than ET-1 at the same concentration. Thus, at threshold the rank order of peptides was ET-1 = SX6b > ET-3 >>> SX6c, and at concentrations of 3 x 10(-8)M and higher, SX6b > ET-3 > ET-1 >>> SX6c. 3 In the presence of BQ-123 or PD 142893 (10(-5)M) the threshold concentrations for ET-1 to augment the twitches were increased 30 fold. In the same conditions neither SX6b nor ET-3 potentiated the responses. The relative activities of the endothelin/sarafotoxin peptides and the effectiveness of the endothelin receptor antagonists are consistent with postjunctional ET(A) receptors mediating these effects. 4 In the transmurally stimulated GPI the endothelin/sarafotoxin peptides produced two effects; an increase in the basal tension of the tissues and an inhibition of the twitch responses. To increase the basal tension the peptides had the order of potency ET-1 > SX6b >> ET-3 = SX6c. These direct effects of ET-1 or SX6b were strongly antagonized (100 fold) by either BQ-123 (10(-5)M) or PD 142893 (10(-5)M). Thus, ET(A) receptors mediate contractions of the GPI induced by these peptides. 5 The endothelin/sarafotoxin peptides were approximately equipotent at depressing twitches of the GPI in response to transmural stimulation (EC50s, 4 x 10(-11) to 1.5 x 10(-10)M). The depressions induced by ET-1 were unaffected by either BQ-123 (10(-5)M) or PD 142893 (10(-5)M). BQ-123 produced a small (three fold) antagonism of the inhibitory effects of ET-3 or SX6c. These results indicate that a receptor of the ET(B) type mediates the inhibitory effects of the endothelin/sarafotoxin peptides on neurotransmission in the GPI. 6 Thus, both ET(A) receptors and ET(B) receptors mediate the effects of the endothelin/sarafotoxin peptides on neurotransmission.