CHARACTERISTICS OF ENDOTHELIN BINDING-SITES IN THE SPINAL-CORD OF SPONTANEOUSLY HYPERTENSIVE RATS

The binding of [I-125]sarafotoxin 6b (SRT 6b) and [I-125]endothelin-1 (ET-1) to endothelin (ET) receptors of neuronal membranes prepared from cerebral cortex and spinal cord of 8-week-old spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) was determined. SHR had significantly higher blood pressure as compared to WKY. Heart rate was similar in SHR and WKY. [I-125]SRT 6b and [I-125] ET-1 bound to the membranes of cerebral cortex and spinal cord at a single, high affinity site. The binding of [I-125]SRT 6b and [I-125]ET-1 in the cerebral cortex and spinal cord membranes was found to be similar in SHR and WKY. Concentration-dependent inhibition of [I-125]ET-1 binding in spinal cord membranes by unlabeled ET- 1, ET-2 and ET-3 was performed. The K(i) values were found to be 2.35 +/- 0.68, 0.29 +/- 0.08 and 24.10 +/- 5.90 nM for ET-1, ET-2 and ET-3, respectively in WKY. The K(i) values of ET-1 and ET-2 were found to be similar in WKY and SHR. However, the K(i) value of ET-3 were found to be significantly lower (P < 0.004) in SHR as compared to WKY. Concentration-dependent inhibition of [I-125]SRT 6b binding in spinal cord membranes by unlabeled ET-1, ET-2 and ET-3 were also performed. The K(i) values were found to be 9.50 +/- 2.10 pM, 0.17 +/- 0.04 nM and 31.20 +/- 6.00 nM for ET-1, ET-2 and ET-3, respectively in WKY. The K(i) values of ET-1 and ET-2 were found to be similar in WKY and SHR. However, the K(i) value of ET-3 were found to be significantly lower (P < 0.01) in SHR as compared to WKY. In the cerebral cortex inhibition of [I-125]SRT 6b and [I-125]ET-1 binding by ET-3 was similar in WKY and SHR. It is concluded that in spinal cord ET receptors are of ET(A) type. It appears that there are two subtypes of ET(A) receptors in the spinal cord, ET(A1) which are more selective to ET-1 as compared to ET-2 and ET(A2) which are more selective to ET-2 as compared to ET-1. In SHR these receptors are more sensitive to ET-3 as compared to WKY and may be involved in the regulation of blood pressure.