Wei-Hua Cao

Renal sympathetic nerve activity in mice: comparison between mice and rats and between normal and endothelin-1 deficient mice

Recently generated knockout mice with disrupted genes encoding endothelin (ET)-1 showed an elevation of arterial blood pressure (AP) and supplied an evidence for intrinsic ET-1 as one of the physiological regulators of systemic AP. Little is yet known, however, why deficiency of ET-1, which was originally found as a potent vasoconstrictor, led to higher AP in these mice. To address this apparent paradox, we first developed a method to measure renal sympathetic nerve activity (RSNA) in mice using rats as reference and successively compared it between normal and ET-1 deficient mice. RSNA was successfully recorded in urethane-anesthetized and artificially ventilated mice by a slight modification of the method used for rats. At basal condition, mean AP (MAP) and RSNA in ET-1 deficient mice (105+/-2 mmHg and 9.71+/-1.49 muVs, n=20) were significantly higher than those in wild-type mice (96+/-2 mmHg and 5. 07+/-0.70 muVs, n=25). Basal heart rate (HR) and baroreflex-control of HR was not significantly different between the two. On the other hand, resting RSNA, RSNA range, and maximum RSNA were significantly greater in ET-1 deficient mice, and thus MAP-RSNA relationship was upwards reset. Hypoxia-induced increase in RSNA was not different between ET-1 deficient (73.4+/-9.4%) and wild-type mice (91.2+/-12.0%), while hypercapnia-induced one was significantly attenuated in ET-1 deficient mice (18.8+/-3.6 vs. 39.1+/-5.2% at 10% CO2). These results indicate that endogenous ET-1 participates in the central chemoreception of CO2 and reflex control of the RSNA. Baroreceptor resetting and normally preserved hypoxia-induced chemoreflex may explain a part of the elevation of AP in ET-1 deficient mice.

Action of endothelin-1 on vasomotor neurons in rat rostral ventrolateral medulla

Intracisternal administration of endothelin-1 (ET-1) elicits sympathetically mediated cardiovascular responses by acting on the ventral surface of the medulla oblongata (VSM) subjacent to the rostral ventrolateral medulla (RVLM). We examined, in urethane-anesthetized rats, whether intracisternal ET-1 affected activity of vasomotor neurons (VMNs) in the RVLM, by acting either directly on the VMNs or indirectly via the VSM. VMNs were identified electrophysiologically. Intracisternal administration of ET-1 altered activity of all the 13 VMNs tested. At a dose of 0.1 pmol, ET-1 invariably caused transient excitation in six VMNs examined, whereas at a dose of 1 pmol in separate experiments all the seven VMNs tested were inhibited with (n = 6) or without (n = 1) preceding excitation. Similarly, topical application of ET-1 (0.1-1 pmol) to the VSM caused inhibition with (n = 3) or without (n = 2) preceding excitation in all the five VMNs tested. Direct iontophoretic application of ET-1 to the VMNs caused excitation in four of seven VMNs examined but did not affect the other three neurons. These results support the view that intracisternally administered ET-1 alters activity of VMNs in the RVLM, by acting directly on neurons themselves and indirectly via the VSM.

Effect of endothelin on vasomotor and respiratory neurons in the rostral ventrolateral medulla in rats

Abstract1. We have previously shown that intracisternal administration of endothelin-1 (ET-1) elicited cardiorespiratory responses acting on the ventral surface of the medulla oblongata (VSM) subjacent to the rostral ventrolateral medulla (RVLM). In this study, we examined whether vasomotor and respiratory neurons in RVLM participate in above-mentioned responses and whether those neurons respond to direct iontophoretic application of ET-1 and/or an ET-A receptor antagonist, FR139317.2. Unit activity of vasomotor, respiratory, or nociceptive neurons in RVLM was recorded together with arterial blood pressure (AP) and heart rate (HR) in urethane-anesthetized Sprague-Dawley rats.3. Intracisternal administration or topical application of ET-1 (0.1–1 pmol) to VSM caused excitation of the majority of vasomotor neurons (15/18) and respiratory neurons (10/11) but not in nociceptive neurons (0/7). Changes in neuronal activity were in similar time course with corresponding changes in AP and HR. Iontophoretic application of ET-1 to the vicinity of recording neuron caused excitation in 19 of 21 vasomotor neurons without affecting AP nor HR. Remaining two neurons were insensitive to ET-1. FR139317 did not affect basal activity of the vasomotor neurons but inhibited ET-1-evoked excitation. Twenty-four of 40 respiratory neurons were excited and 13 were inhibited by iontophoretic application of ET-1. Five of ET-1-excited respiratory neurons were inhibited by FR139317 alone while six of ET-1-inhibited neurons were not affected by FR139317 alone. In both cases, FR139317 inhibited the effect of simultaneously applied ET-1. Iontophoretic application of ET-1 excited only one out of 10 nociceptive neurons so far tested.4. These results support the view that intracisternally administered ET-1 alters activity of vasomotor and respiratory neurons in the RVLM, at least in part by acting directly on neurons themselves and hence causes systemic cardiorespiratory changes. Majority of vasomotor and respiratory neurons should express ET-A receptors and some respiratory neurons are under tonic excitatory control by ET-1.

Endothelin-sensitive areas in the ventral surface of the rat medulla

In urethane-anesthetized rats, subregions of the ventral surface of the medulla (VSM) in which endothelin (ET) caused cardiorespiratory effects were mapped by topically applying 1 pmol of ET-1. Two distinct subregions, termed the rostral and caudal ET-sensitive areas, were identified. The rostral area was also sensitive to L-glutamate and glycine. It extended between the caudal end of the trapezoid body and the rootlet of the XIIth nerve partly overlying the pyramidal tract. In this position ET-1 caused the type I response consisting of an initial increase (excitatory component) in arterial pressure (AP), renal sympathetic nerve activity (RSNA), heart rate (HR), phrenic nerve activity (PNA) and the number of bursts of PNA (burst rate) followed by a sustained decrease (inhibitory component) in them. The caudal ET-sensitive area was located near the rootlet of the XIIth nerve. In this position ET-1 caused the type II response consisting of a decrease in PNA and an increase in burst rate. Part of this area responded to nicotine but not to glutamate or glycine. ET-3 (10 pmol) applied to the two ET-sensitive areas produced responses similar to those elicited by ET-1. The dose-response relationship was investigated by delivering ETs to the rostral area. The excitatory component of most of the variables was elicited at a dose of 1 fmol of ET-1 or 1 pmol of ET-3, whereas the inhibitory component was produced at 10 fmol of ET-1 or 10 pmol of ET-3. These results suggest that subregions of the rats VSM may participate in the central cardiorespiratory control by ET.

High calcium diet prevents baroreflex impairment in salt-loaded spontaneously hypertensive rats.

To investigate the role of the sympathetic control mechanism in the antihypertensive effect of dietary calcium supplementation, we examined whether a high calcium diet affected mean arterial pressure, renal sympathetic nerve activity, heart rate, and overall and central properties of the arterial baroreceptor reflex in salt-loaded young spontaneously hypertensive rats (SHR). Six-week-old SHR were fed either a normal (0.66%) or high (8.00%) salt diet with either a normal (1.17%) or high (4.07%) calcium content for 4 weeks. The arterial baroreceptor reflex was elicited with rats under halothane anesthesia by altering mean arterial pressure with nitroprusside or phenylephrine. The overall property of the arterial baroreceptor reflex was assessed by the median mean arterial pressure (MAP50) and maximal gain (Gmax) of the relation between mean arterial pressure and renal sympathetic nerve activity and between mean arterial pressure and heart rate. The central property of the arterial baroreceptor reflex was assessed by reflex inhibition of renal sympathetic nerve activity and heart rate elicited by electrical stimulation of the aortic depressor nerve. Compared with the control group fed a normal salt/normal calcium diet, the high salt/normal calcium group had significantly higher mean arterial pressure and renal sympathetic nerve activity but not heart rate. Moreover, the arterial baroreceptor reflex was impaired in the latter group, as evidenced by an increase in MAP50 and decrease in Gmax of the two relations and an attenuation of reflex inhibition of renal sympathetic nerve activity by aortic depressor nerve stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiorespiratory effects of topical application of endothelin-1 to the ventral surface of the rat medulla

In urethane-anesthetized and vagotomized rats, we examined the cardiorespiratory effects of a topical application of endothelin-1 (ET-1) to the ventral surface of the medulla (VSM) and surveyed subregions of the VSM influenced by these effects. ET-1 (0.1 fmol) delivered to the S area of the VSM via a needle (i.d. of approximately 100 microns) gently pressed on the VSM had no effect on mean arterial pressure (MAP), heart rate (HR), renal sympathetic nerve activity (RSNA), phrenic nerve activity (PNA), or the burst rate of PNA. However, a dose of 1 fmol of ET-1 induced transient but significant increases in MAP, HR, RSNA, and burst rate while at a dose of 10 fmol or more, PNA also increased and simultaneously longer-lasting decreases in MAP, RSNA, and PNA followed the initial increase. The subregion of the VSM in which ET-1 most prominently elicited these effects was the S and caudal part of the M area, where topical application of 50 nmol of L-glutamate caused cardiorespiratory changes. Additionally, there was a restricted region within the caudal VSM in which ET-1 caused a decrease in PNA with an increase in burst rate. These results support our hypothesis that the VSM is crucially involved in the cardiorespiratory changes induced by centrally administered ET-1.