Cardiovascular and respiratory responses to chronic intermittent hypoxia in adult female rats

Souza GM, Bonagamba LG, Amorim MR, Moraes DJ, Machado BH.
Exp Physiol. 2014 Dec 16. doi: 10.1113/expphysiol.2014.082990

Volumes of research indicate that males are more susceptible to developing hypertension than premenopausal females. There is also data that shows that obstructive sleep apnea, a form of chronic intermittent hypoxia (CIH), leads to hypertension and that this apnea is more common in men than women. However, data linking this apnea to differences in men and women is scarce and a bit controversial.  In this study they looked at female rats exposed to CIH and examined changes in breathing patterns and blood pressure.
In this paper, they used a different CIH protocol than what was in some of the other papers I recently blogged. Here, they did 9 minutes of normoxia, and used nitrogen displacement to bring oxygen levels down to 6% for 40seconds, before being brought back to normoxia for another 9 minutes. This continued for 8hrs a day for 35 days. Rats were anesthetized and fitted with a femoral catheter to record arterial pressure. Respiratory activity was recorded 30 minutes after placing the rat inside of the plethysmographic chamber.
What they noticed was that CIH really messed up female rats’ cardiovascular system - they weighed less than controls (273 vs 347g), they had higher systolic (136 vs 129mmHg), diastolic (92 vs 86mmHg), and mean arterial pressures (111 vs 104mmHg), as well as increased heart rate (400 vs 376bpm). The systolic pressure was also more variable in CIH females. However, the dramatic changes seen in the cardiovascular system was not reflected in the respiratory system. Compared to controls, CIH rats didn’t have higher minute volume, tidal volume, or respiration frequency. However, short and long term variability was indeed higher in CIH rats.
Because respiratory and cardiovascular systems are coupled, they looked at how this relation changed between rat groups.  CIH rats had a stronger fall in MAP during deep breaths, and were more likely to have an apneic event after a deep breath than control rats, and the apnea lasted longer as well. They noted that their results were very comparable to the results of a different study on male rats that used the same protocol, but conflicted with another study in female rats that used a different CIH protocol which said that females DO NOT experience the strong changes seen in this paper. This suggests that females may have some protection against the changes, presumably due to differences in hormone levels, but that protection can be overcome by more severe CIH.  -DH

Neural control of the circulation: how sex and age differences interact in humans.

Joyner MJ, Barnes JN, Hart EC, Wallin BG, Charkoudian N.

Compr Physiol. 2015 Jan 1;5(1):193-215.

For reasons I am probably not at liberty to discuss, some of us in the lab have been discussing the differences in autonomic function between males and females. So I was pretty happy to see a review that had some of this information in it. As an added bonus, it discusses these differences mostly in humans (because we all need to focus on the transnational application of our basic science) with animal data where no human data was available.  Added bonus 2 is that this review comes from some reputable people, so it’s well-written and easy to understand.

In the review, they touched on a lot of topics, ranging from how (muscle) sympathetic nerve activity may be correlated with blood pressure in any given young man, but not from young man to young man, and not at all in young women. They also included various difficulties in experimental techniques (conduction speed differences between sympathetic and parasympathetic systems), neurotransmitter turnover times, and receptor availability/activity with modulation by different factors.  A couple of interesting things I learned were that even though estrogen is widely thought to prevent high SNA and blood pressure, conflicting data has also shown that high estrogen can sometimes correlate with high SNA, and that the age related changes seen in women after menopause will also occur regardless of menopause, and may just be related to age.  Confusing stuff.

Anyway, I liked this review a lot. I were running a lab, I would probably add it to the list of early required reading for new students. –DH

Chronic intermittent hypoxia increases sympathetic control of blood pressure: role of neuronal activity in the hypothalamic paraventricular nucleus.

Sharpe AL, Calderon AS, Andrade MA, Cunningham JT, Mifflin SW, Toney GM.

Am J Physiol Heart Circ Physiol. 2013 Dec;305(12):H1772-80.

This is a very slightly older paper from some of the same people I posted about last weekend. They were still looking to see how chronic intermittent hypoxia (CIH) could affect lumbar and renal sympathetic nerve activity (LSNA and RSNA), based on the existing theory that CIH causes hyperactivity of the chemoreflex, and the changes happen by way of the paraventricular nucleus (PVN). This time, the parameters of the CIH were: O2 levels were reduced to 10% for 6 minutes, 10x per hour, for 8 hours a day, for 7 consecutive days.

In this study they found that, compared to control rats, CIH rats had significantly higher MAP by day 3, with differences in heart rate not reaching significance. Rats exposed to 7 days of CIH had a greater reduction in MAP after ganglionic blockade (hexamethonium) than was seen in control rats, which indicated that CIH rats have increased sympathetic tone. Furthermore, muscimol inactivation of PVN caused greater reductions of lumbar SNA and MIP in CIH rats than controls, which also supports the idea that CIH rats have higher sympathetic tone. Furthermore, when CIH rats were given hypertonic saline directly in to the carotid artery, they showed a larger increase in LSNA (but not in RSNA) than control rats did. This suggests that there may be a greater LSNA responsiveness in CIH rats, and NOT a blunted/ceiling effect due to the higher SNA tone. This is also interesting on the basis that it adds more data to the idea of differential control of SNA. -DH

Adenosine reduces GABAergic IPSC frequency via presynaptic A1 receptors in hypothalamic paraventricular neurons projecting to rostral ventrolateral medulla

Tae Hee Han,Soo Hwa Jang,So Yeong Lee,Pan Dong Ryu. Neuroscience Letters

Volume 490, Issue 1, 18 February 2011, Pages 63–67. doi:10.1016/j.neulet.2010. 12.026. The paraventricular nucleus (PVN) is a brain region that has projections that go to RVLM and down to the IML in order to modulate sympathetic outflow. The purpose of this study was to determine whether adenosine was was playing a role in modulating gaba release from PVN-RVLM neurons. Using young male Sprague-Dawley they labelled PVN-RVLM neurons by injecting Fluospheres- Red into the RVLM and they allowed the animal to recover for 5-7 days. the brain was sectioned and the labelled PVN neurons were selected for recording. In response to adenosine, they saw inhibitory postsynaptic currents . When compared to the before activity there was a decrease in firing but not in amplitude and the response was concentration dependent. Then they used antagonist to A 1 receptor and the A2  receptor  and saw that there was no change in the firing of the neuron. however, when adenosine was given  after the  microinjection of the A1 antagonist they found that the iPSCs was inhibited.  the microinjection of  the A2 antagonist  did not prevent the IPSCs that occur in response to  adenosine injection. These data demonstrate that adenosine is playing a  role presynaptically  in attenuating GABA release and this mediated by adenosine acting on A1 receptors. -MD

Acute intermittent optogenetic stimulation of nucleus tractus solitarius neurons induces sympathetic long-term facilitation.

Yamamoto K, Lalley PM, Mifflin SW.

Am J Physiol Regul Integr Comp Physiol. 2014 Dec 17:ajpregu.00381.2014

                In this paper, they built on previous work that showed that the effect of acute intermittent hypoxia (AIH) on long term facilitation (LTF) of phrenic nerve activity (PNA) and renal sympathetic nerve activity (RSNA) seems to be routed through the nucleus tractus solitarius (NTS), and that you could induce the same effect by electrical stimulation of the carotid sinus, even without hypoxia. They took this idea a step further by causing expression of channelrhodopsin in the caudal NTS, via a virus that would cause preferential expression in glutamatergic neurons.

                Once they had this system in operation, they compared the effects of AIH with the effects of acute intermittent optogenetic stimulation (AIO) on RSNA and PNA immediately after, and 60 minutes after periods of stimuli. They found that AIO in the caudal NTS produces a similar, but weaker, response to that seen after AIH (RSNA and PNA increased by 60% and 100% after AIO, but by 80 and 130% after AIH).  They also found that, while both stimuli increased the power spectral density of RSNA and PNA at their own primary frequencies, neither stimulus was able to increase synchronization of PNA with RSNA. -DH

Activation of Corticotropin Releasing Factor Receptors in the Rostral Ventrolateral Medulla is Required for Glucose-Induced Sympathoexcitation

Megan E. Bardgett, Amanda L. Sharpe, Glenn M. Toney.Am J Physiol Endocrinol Metab (September 30, 2014). doi:10.1152/ajpendo.00291.2014.

Glucose leads to increased energy expenditure  through activation sympathetic nerve activity (SNA). The mechanism that leads to the activation of SNA is unknown. PVN and RVLM play an important role in the control of SNA and BP.  We already know that PVN not directly projects down to the IML in order to control SNA and BP but there is a direct projection to RVLM from PVN. Also neurons in the PVN are activated by glucose. The activity of RVLM is controlled mainly by Glutamate and GABA but we also know that there are other neurotransmitters that may be involved such as corticotropin releasing factor (CRF). So it has been shown that there are CRFergic neuron in the PVN. The hypothesis for the study in this article was that glucose leads to increases in SNA through activation of CRFergic neurons in the PVN that ultimately lead to activation of RVLM neuron by activating CRF receptor that lead to sympathoexcitation.  In order to prove this they provided anatomical along with function data. The data provided showed that glucose infusion elevates both LSNA and SSNA. They also demonstrated that glucose does, in fact, activate neurons in both RVLM and PVN. Not only that but the majority of PVN neurons that expressed c-fos was also a CRFergic neuron. As for RVLM, they showed that glucose activated a significant portion of TH RVLM neurons. Blockade of CRF receptor blunted the increases in LSNA and SSNA in response to glucose infusion. They also showed that kyn blocks the increase in SNA in response to glucose. Final they demonstrated that blockade of PVN blocked the response to glucose infusion. These data demonstrate that glucose leads to activation of SNA by activating CRFergic neurons in the PVN and this leads to the release of CRF and this leads to activation or the CRF receptor and this may facilitate the release of glutamate in the RVLM and this leads to increases in SNA and thus energy expenditure. I wonder how SNA in our model might behave in response to glucose infusion…-MD

CNS neuroplasticity and salt-sensitive hypertension induced by prior treatment with subpressor doses of ANG II or aldosterone

Sarah C. Clayton, Zhongming Zhang, Terry Beltz, Baojian Xue, and Alan Kim Johnson.

Am J Physiol Regul Integr Comp Physiol 306: R908–R917, 2014.First published April 2, 2014; doi:10.1152/ajpregu.00010.2014. Hypertension is obviously a problem in today’s society.  About 25% of all hypertensive patients are salt sensitive. However the mechanism that is responsible for salt sensitivity leading to hypertension is still not clearly understood. It has been shown that pretreatment with  nonpressor doses of ANG II and aldosterone can lead to enhanced responses to ANG II. This sensitization depends on a functional brain renin angiotensin aldosterone system. Long term changes in the CNS could occur in response to sensitization. This studied investigated whether BDNF a protein that plays a role in many models neuroplasticity is involved in the changes that are occurring in the lamina terminalis (LT) in response to sensitization to ANGII.  the study showed that pretreatment to ANGII or Aldosterone in the brain can lead sensitization to salt. They also showed that the BDNF is enhanced along with p38 MARK and pCREB in response to pretreatment to ANGII and aldosterone. These data demonstrate that pretreatment with subpressor doses of ANGII and aldosterone can lead long term structural changes by increasing the the BDNF pathway in LT and this contributes to the development of salt sensitivity. I wished they would have investigated the RVLM, wouldn’t that be interesting.-MD