Gen. Pharmac., 1976, VoL 7, pp. 35 to 40. Pergamon Press. Printed in Great Britain
C I R C A D I A N RHYTHMS IN BLOOD PRESSURE A N D N O R E P I N E P H R I N E IN GENETICALLY HYPERTENSIVE A N D NORMOTENSIVE RATS* G. M. LEw Department of Anatomy, Michigan State University, East Lansing, MI 48824, U.S.A.
(Received 28 July 1975) Abstract--1. Circadian changes in systolic blood pressure (B.P.) were deterrnined in genetically hypertensive (GI-I) and normotensive rats and correlated with changes in norepinephrine (NE) levels in various brain regions, the heart and the adrenal glands. 2. In the hypothalamus of GH rats a relatively low level of NE was found when the B.P. reached a peak level during the late dark phase, and a relatively high level of this catecholamine was found when the B.P. was low in the light period. 3. The most significant difference between hypertensive and control rats in NE concentration in the brainstem and heart was found in the early light hours, when the B.P. was relatively low. Previous studies have shown a possible deviation in central norepinephrine (NE) metabolism in spontaneously hypertensive rats. The possible role of a central noradrenergic mechanism in the modulation of blood pressure is in need of clarification. A deficiency of N E in the brainstem has been considered as one of the possible factors in the pathogenesis of genetic hypertension. However, no direct correlation has as yet been made between blood pressure and the level of catecholamine biosynthetic enzymes in the brainstem (Yamori et al., 1970, 1972b and 1973). Since much evidence has been accumulated on circadian rhythms in catecholamines in various brain regions (Reis et al., 1968; Walker et al., 1971; Morgan et aL, 1973 and Lew & Quay, 1973a), the the effects of possible circadian changes in N E content and metabolism must be considered in the design of experiments on hypertensive rats. Information on these rhythms can also lead to new therapeutic approaches. The aims of the present investigation were to determine circadian changes in systolic blood pressure (B.P.) in genetically hypertensive (GH) and normotensive rats and to correlate them with circadian changes in N E concentration in regions of the brain as well as in the heart and adrenal glands. MATERIALS AND METHODS
Genetically hypertensive and normotensive rats (New Zealand strain, Phelan, 1968; Lew, 1975) were bred in one of two windowless, air-conditioned rooms at approximately 22°C. The rooms were maintained under artificial illumination (one room frold 6 a.m. to 8 p.m., EST. and the other from 2 p.m. to 4 a.m., EST, each day), The rats were housed in large plastic cages and were fed on Wayne Lab Blox (Allied Mills) and drinking water
ad libitum.
The systolic blood pressure (B.P.) of each rat was measured by a tail-cuff method with the aid of a NarcoBiosystems pneumatic pulse transducer without the use of anaesthesia. B.P. recordings were made on each rat at an approximate age of 4 months on at least three separate occasions and at the same time each day at twoto three-day intervals. Blood pressure measurements were again made on each rat at an approximate age of 5½ months in a similar manner prior to the beginning of the experiment. Six groups of GH rats were selected. Six groups of age- and weight-matched normotensive rats served as controls. The five rats in each of the 12 groups (one rat for each of the 5 times in 24 hr at which B.P. measurements were made) had similar B.P. measurements. During the experimental period, on three separate occasions, at two- to three-day intervals, B.P. measurements were made on each rat in each group. On the day following that on which the last B.P. recording was made on each group, the 5 rats were sacrificed (one rat at each of the 5 times in 24 hr at which the B.P. was taken by the same individual). All rats were quickly decapitated; dissection boundaries of the brain regions, removal of organs, freezing of tissues, and extraction and fluorometric determinations of catecholamines were as described previously (Lew & Quay, 1973a). The significance of the difference between mean values was determined by the Student-Fisher t-test. RESULTS
Circadian rhythms in systolic blood pressure The daily mean systolic B.P. of the normotensive rats was 1304-4 mm. Hg while that of the G H rats was 176-4-5 m m I-Ig. The mean B.P. o f the G H rats was significantly greater than that of control rats
* This research was supported by a grant (ORD 13495) from the Michigan Heart Association. 35
36
G . M . LEw
Table 1. Twenty-four-hour changes in systolic blood pressure (B.P.) in genetically hypertensive (GH) and normotensive rats SYSTOLICBLOODPRESSURE(mnflg)
TII~E (clock hours)
Normotensive 0700
172 t 4**(A)
135 ± 3 (B)
llO0
157 ± 2"*(C)
121 ± 2**(D)
1700
171 ~ 5**(E)
123 * 3**(F)
0100
174 ± 2**(G)
120 ± 2* (H)
0500
.~> 196 ~ 5 ( I )
.--~; 143 ± 3 (J)
Ph-o~operiod: 0600 - 2000 (EST) Values are means ± SE. **p Z .01, *PC .05 with reference to noted ('-'->) group. A-B:P< .01; C-D:PK.OOl; E-F:P< .05; G-H:P4 .001; I-J;P4.00l. Daily meanB.P. = 176 ~ 5 for GH rats and 130 ~ 4 for normotensive rats.
at each of the five times in 24 hr when B.P. recordings were made (Table 1). The most striking variation in pressure was seen during the late dark phase when a dramatic increase in pressure occurred in both G H and control rats (Fig 1). In G H rats this 17
21
01
05
07
II
17
increase was 11 ~ above the daily mean and significantly greater (P
C I R C A D I A N RHYTHMS IN BLOOD PRESSURE A N D N O R E P I N E P H R I N E IN GENETICALLY HYPERTENSIVE A N D NORMOTENSIVE RATS* G. M. LEw Department of Anatomy, Michigan State University, East Lansing, MI 48824, U.S.A.
(Received 28 July 1975) Abstract--1. Circadian changes in systolic blood pressure (B.P.) were deterrnined in genetically hypertensive (GI-I) and normotensive rats and correlated with changes in norepinephrine (NE) levels in various brain regions, the heart and the adrenal glands. 2. In the hypothalamus of GH rats a relatively low level of NE was found when the B.P. reached a peak level during the late dark phase, and a relatively high level of this catecholamine was found when the B.P. was low in the light period. 3. The most significant difference between hypertensive and control rats in NE concentration in the brainstem and heart was found in the early light hours, when the B.P. was relatively low. Previous studies have shown a possible deviation in central norepinephrine (NE) metabolism in spontaneously hypertensive rats. The possible role of a central noradrenergic mechanism in the modulation of blood pressure is in need of clarification. A deficiency of N E in the brainstem has been considered as one of the possible factors in the pathogenesis of genetic hypertension. However, no direct correlation has as yet been made between blood pressure and the level of catecholamine biosynthetic enzymes in the brainstem (Yamori et al., 1970, 1972b and 1973). Since much evidence has been accumulated on circadian rhythms in catecholamines in various brain regions (Reis et al., 1968; Walker et al., 1971; Morgan et aL, 1973 and Lew & Quay, 1973a), the the effects of possible circadian changes in N E content and metabolism must be considered in the design of experiments on hypertensive rats. Information on these rhythms can also lead to new therapeutic approaches. The aims of the present investigation were to determine circadian changes in systolic blood pressure (B.P.) in genetically hypertensive (GH) and normotensive rats and to correlate them with circadian changes in N E concentration in regions of the brain as well as in the heart and adrenal glands. MATERIALS AND METHODS
Genetically hypertensive and normotensive rats (New Zealand strain, Phelan, 1968; Lew, 1975) were bred in one of two windowless, air-conditioned rooms at approximately 22°C. The rooms were maintained under artificial illumination (one room frold 6 a.m. to 8 p.m., EST. and the other from 2 p.m. to 4 a.m., EST, each day), The rats were housed in large plastic cages and were fed on Wayne Lab Blox (Allied Mills) and drinking water
ad libitum.
The systolic blood pressure (B.P.) of each rat was measured by a tail-cuff method with the aid of a NarcoBiosystems pneumatic pulse transducer without the use of anaesthesia. B.P. recordings were made on each rat at an approximate age of 4 months on at least three separate occasions and at the same time each day at twoto three-day intervals. Blood pressure measurements were again made on each rat at an approximate age of 5½ months in a similar manner prior to the beginning of the experiment. Six groups of GH rats were selected. Six groups of age- and weight-matched normotensive rats served as controls. The five rats in each of the 12 groups (one rat for each of the 5 times in 24 hr at which B.P. measurements were made) had similar B.P. measurements. During the experimental period, on three separate occasions, at two- to three-day intervals, B.P. measurements were made on each rat in each group. On the day following that on which the last B.P. recording was made on each group, the 5 rats were sacrificed (one rat at each of the 5 times in 24 hr at which the B.P. was taken by the same individual). All rats were quickly decapitated; dissection boundaries of the brain regions, removal of organs, freezing of tissues, and extraction and fluorometric determinations of catecholamines were as described previously (Lew & Quay, 1973a). The significance of the difference between mean values was determined by the Student-Fisher t-test. RESULTS
Circadian rhythms in systolic blood pressure The daily mean systolic B.P. of the normotensive rats was 1304-4 mm. Hg while that of the G H rats was 176-4-5 m m I-Ig. The mean B.P. o f the G H rats was significantly greater than that of control rats
* This research was supported by a grant (ORD 13495) from the Michigan Heart Association. 35
36
G . M . LEw
Table 1. Twenty-four-hour changes in systolic blood pressure (B.P.) in genetically hypertensive (GH) and normotensive rats SYSTOLICBLOODPRESSURE(mnflg)
TII~E (clock hours)
Normotensive 0700
172 t 4**(A)
135 ± 3 (B)
llO0
157 ± 2"*(C)
121 ± 2**(D)
1700
171 ~ 5**(E)
123 * 3**(F)
0100
174 ± 2**(G)
120 ± 2* (H)
0500
.~> 196 ~ 5 ( I )
.--~; 143 ± 3 (J)
Ph-o~operiod: 0600 - 2000 (EST) Values are means ± SE. **p Z .01, *PC .05 with reference to noted ('-'->) group. A-B:P< .01; C-D:PK.OOl; E-F:P< .05; G-H:P4 .001; I-J;P4.00l. Daily meanB.P. = 176 ~ 5 for GH rats and 130 ~ 4 for normotensive rats.
at each of the five times in 24 hr when B.P. recordings were made (Table 1). The most striking variation in pressure was seen during the late dark phase when a dramatic increase in pressure occurred in both G H and control rats (Fig 1). In G H rats this 17
21
01
05
07
II
17
increase was 11 ~ above the daily mean and significantly greater (P
