Philadelphia, PA - Results from a new study have shown that various genes that play a role in the molecular clock also regulate enzymes relevant to the synthesis and disposition of catecholamines, resulting in alterations in blood pressure and plasma norepinephrine and epinephrine throughout the day, as well as changes in these variables in response to stress [1]. The findings provide evidence for two apparently conflicting explanations for time-dependent changes in blood pressure, say investigators.

"What we found was that different transcription factors involved in the core clock had an impact on blood pressure that was quite distinct and that there was indeed a temporal conditioning of the response to stress," said senior investigator Dr Garret FitzGerald (University of Pennsylvania, Philadelphia). "We also made this very surprising observation that if we knocked out the most nonredundant transcription factor implicated in the clock, we seem to impact on an aspect of the stress response that is quite distinct from time. As we tried to look for a mechanism that brought clock genes into play, in terms of diurnal variation, we found that several genes involved in the catecholamine pathway were also under control of the molecular clock."

The results of the study, published online February 21 in the Proceedings of the National Academy of Sciences with lead author Dr Anne M Curtis (University of Pennsylvania), raise the possibility of modifying blood pressure and the subsequent morning risk of cardiovascular events by resetting the molecular clock, FitzGerald told heartwire. As this blood-pressure response is linked to the internal clock, new means of "phase-shifting" the time of the maximal increase in blood pressure could potentially decrease the risk of early-morning MI and stroke, he said.

The molecular clock for mammals is located in the suprachiasmatic nucleus of the brain and is made up of positive and negative transcriptional and translational feedback loops that drive circadian gene expression. Although the clinical onset of vascular events occurs more frequently in the morning than at any other time of day and this circadian variation corresponds to the early-morning increase in blood pressure, it is unknown whether this increase in blood pressure is the result of the molecular clock or merely reflects the physical and emotional stress of getting up and getting active after a healthy slumber.

To address this question, the researchers used mouse models in which various core clock genes—Bmal1, Clock, and Npas2—were disrupted, and they discovered distinct and complementary effects of these genes on blood pressure and its circadian variation. In addition, the researchers also showed that genes involved in the production and catabolism of catecholamines were also under control of the molecular clock. They subjected the mice to a standardized stress in response to which catecholamines and blood pressure surge. The researchers found that the increase in blood pressure and catecholamines depended on the time of the stress and that the largest increase occurred at a time that coincided with the early morning in humans.

"We turned our attention to that particular pathway, as we know that plasma norepinephrine and epinephrine undergo a diurnal variation that tracks exactly with blood pressure," said FitzGerald. "We know also that this system—the 'fight-or-flight response'—is turned on in response to stress, and so, if you like, there is a mechanistic commonality between the two."

Investigators also showed that deleting one of the core clock genes abolished both the catecholamine and blood-pressure response to stress, regardless of when the stress was applied throughout the day. This effect was specific only to the catecholamines, as the stress response of another hormone, a steroid, was unchanged.

"While these different transcription factors impacted on blood pressure, we found that they also had a similar hierarchy of impact on plasma catecholamines," explained FitzGerald. "When we knocked out the Bmal1 transcription factor, we found an effect on the stress response that was independent of time, that the part of the stress response it was affecting was the catecholamines and the consequent rise in blood pressure. What was left untouched was the steroid response to stress."

Conservation of the steroid response was dramatic but consistent with earlier results from the group in a different setting. Previous studies, said FitzGerald, have shown a role for these clock genes in regulating the recovery from insulin-induced hypoglycemia [2]. They had assumed that this occurred by abolishing the counterregulatory hormone response—glucagon and steroids—that are turned on in response to hypoglycemia. However, the steroid response was left untouched, and the result reflected control by the clock of a series of genes relevant to gluconeogenesis. This most recent study is a second example of the conservation of the steroid response, but in this particular example, it is maintained with the ablation of the catecholamine response to stress.

"The nice thing about this present study is that it molecularly integrates the two apparently conflicting explanations for time-dependent changes in blood pressure," said FitzGerald.