The Relationship between High-salt and Estrogen on Blood Pressure under Basal and Stressed conditions in the female offspring of Sprague Dawley Rats that were Subjected to High or Normal Salt Diets during Pregnancy

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Brandon G. Wilkinson and Dr. James Porter, Physiology and Developmental Biology

Cardiovascular disease (CVD) is the number one leading cause of death in the United States. High blood pressure (hypertension) is a form of CVD and affects nearly 1 in 3 American adults. Men are more likely than women to develop hypertension until 45 years of age. However, percentages of women diagnosed with hypertension rise to exceed that of men around 45-54 years of age.1 This trend in hypertension is a clear age-related sex dimorphism. The trend suggests that following the onset of menopause, women have a higher risk of hypertension, a risk that continues to rise with age.2-3 Current research with rats have shown that female offspring of rats fed a high-salt diet during pregnancy exhibit a higher blood pressure response to stress than male offspring and those offspring of rats fed a normal salt diet during pregnancy.4 Other studies have discovered that removing the ovaries from female intra-uterine growth restricted (IUGR) rats caused them to have a similar increase in blood pressure response when stressed.5 Given the above facts, it appears that estrogen or some function of the ovary may be playing a role in protecting females from developing hypertension.

Based on the above research, I hypothesized that something in the ovaries (estrogen?) is protecting against hypertension in females. Furthermore, my goal was to identify the relationship(s) between a high-salt diet and ovary function on blood pressure under basal and stressed conditions. I hypothesized that removal of the ovaries would cause an increase in blood pressure and heart rate in response to stress.

To test my hypothesis, Female Sprague Dawley rats were allowed to mate and become pregnant. The pregnant rats were divided into two groups: half of the rats were subjected to a normal salt diet (.7% NaCl) and half subjected to a high-salt diet (8% NaCl) for the entire pregnancy. The female offspring of these rats were then taken and studied as adults. Hereafter, female offspring of rats subjected to a normal salt diet will be denoted NS rats and the female offspring of rats fed a high-salt diet will be denoted HS rats. After birth, both NS and HS rats were taken from their mothers and fed a normal salt diet for 10 weeks (long enough to reach maturity). At 10 weeks, the ovaries were removed from one-half of all NS and HS rats (hereafter denoted NSovx and HSovx respectively). The remaining NS and HS rats in each group underwent a sham ovariectomy procedure in which surgery was performed, but the ovaries are left intact (hereafter denoted NSovxsham and HSovxsham). At 12 weeks, a radio-telemetry probe was inserted into the abdominal cavity of each rat in order to measure mean arterial blood pressure (MAP) and heart rate (HR). Following a 7-day recovery, MAP and HR were continually measured for 14 days. The rats were then placed in restraining cages and HR and MAP levels were recorded for a period of 30 minutes of stress.

When testing the four groups of rats (NSF OVX, HSF OVX, NSF Sham, and HSF Sham) results showed no significant differences in MAP when the measurements from the four different groups were compared. Most importantly, NS OVX rats did not show a significant drop in MAP like was expected. Furthermore, results also showed no significant differences in MAP when the results from HSF OVX and NSF OVX were compared. Lastly, HS OVX and HS sham rats also showed no significant differences when measurements were compared to one another. Therefore, based on the results it seems that removal of the ovaries has no affect and does not alter MAP levels in any of the four groups in response to stress.

Tests on heart rate (HR) however, showed some interesting results. When NSF Sham rats were compared with NSF OVX rats, results showed that NSF OVX rats seemed to be hypo-responsive to stress with a p-value of 0.058. Although not significant at this value, it is probable that increasing the number of rats in the study would likely produce a significant p-value. Interestingly, these results show the exact opposite of what we had initially hypothesized. Because our results showed opposite of our hypothesis, questions arise as to why HS rats respond differently than NS rats in response to stress.

By comparing the results taken from the HSF OVX and NSF OVX rats, we also discovered that removal of the ovaries causes a smaller drop in HR in the HSF animals as compared to the NSF rats. Interestingly, HR in HSF OVX rats does not mimic the characteristic drop that is seen in NSF OVX rats when subjected to stress. From this we see that something is causing HSF OVX rats to respond to stress differently than NSF OVX rats. Although further study is needed to uncover the reasons for these observations, it is likely that the HS diet may be causing HSF OVX rats to respond differently. Lastly, when HR measurements from HSF sham rats were compared with that of HSF OVX rats, there were no significant differences despite removal of the ovaries. These results suggest that removal of the ovaries in HSF animals has no significant affects on HR.

Although the results of this study did not confirm my hypothesis, they have uncovered some interesting trends that lead us to other hypotheses and further studies. A revised hypothesis might be that the HS diet alters something in the HSF rats that causes them to maintain a higher HR in response to stress than NSF rats. Uncovering the cause of these differences would be a beneficial and practical next step for furthering this study.

References

  • Rosamond W, Flegal K, Friday G, Heart disease and stroke statistics–2007 update: a report from the American Heart Association Statistics Committee. Circulation. 2007;115:e69–e171.
  • Reckelhoff JF. Gender differences in the regulation of blood pressure. Hypertension. 001;37:1199 –1208.
  • Preston RA. Effects of blood pressure reduction on cardiovascular risk estimates in hypertensive postmenopausal women. Climacteric. 2007;10(suppl 1):32– 41.
  • Porter JP, King SH, Honeycutt AD. Prenatal high-salt diet in the Sprague-Dawley rat programs blood pressure and heart rate hyper responsiveness to stress in adult female offspring. Am J Physiol Regul Integr Comp Physiol. 2007 Jul;293(1):R334-42.
  • Alexander BT, Ojeda NB, Grigore D, Robertson EB. Estrogen and increased blood pressure in postpubertal female growth restricted offspring. Hypertension. 2007;50:679-685.