Evaluation of the potential interaction between NaCl and prostaglandin inhibition in elderly individuals with isolated systolic hypertension

Abstract

Objective: To evaluate whether prostaglandin inhibition with the non-steroidal anti-inflammatory drug (NSAID), indomethacin (I) interacts synergistically with different doses of salt (NaCl) in elevating systolic blood pressure (SBP).

Design and methods: This randomized, placebo-controlled, double-blind, crossover study examined the interaction between NaCl and the prostaglandin inhibitor, I in 31 healthy elderly individuals with a mean age (+/- SD) of 68.7+/-5.7 years (range 61-85 years). Participants aged more than 60 years on a 140 mmol/day NaCl dose for 6 weeks were chosen with normal blood pressure [24-h SBP <148 mm Hg, diastolic blood pressure (DBP) <85 mm Hg on the Takeda Ambulatory Blood Pressure Monitor (TABPM); n = 15] and isolated systolic hypertension (ISH), [24-h SBP >148 mm Hg, 24-h DBP <85 mm Hg on TABPM; n = 16]. Exclusion criteria included uncontrolled hypertension (SBP >220 mm Hg and/or DBP >110 mm Hg), cardiac disease, creatinine clearance <60 ml/min, dementia and recent cerebrovascular accident or secondary hypertension. A 2x2 Latin square design was structured using four treatment groups [low salt (NaCl = 90 mmol/day) + I placebo, high salt (NaCl = 240 mmol/day) + I placebo, low salt + I (25 mg three times daily) and high salt + I] for 2 weeks each, balanced and interspersed with 2 week washout periods to minimize carryover effects. Twenty-four hour SBP, DBP and heart rate were measured and summarized using a moving interval averaging technique. The mean change in 24-h SBP, DBP, heart rate, urinary Na+, K+, protein and creatinine, creatinine clearance and serum electrolytes were compared across treatments in the total cohort and in ISH and control groups separately using ANCOVA (SAS).

Results: In the total cohort, compared with low NaCl, chronic high NaCl increased mean SBP (5.76 mm Hg; P = 0.0002) and DBP (3.36 mm Hg; P = 0.002). Indomethacin significantly increased mean SBP (2.66 mm Hg, P = 0.015) but not DBP (0.31 mm Hg, P = 0.419). High salt and I were additive (SBPT, DBPT) but there was no interaction (P = 0.795 and P = 0.739, respectively). Additionally, chronic high NaCl increased serum Na (P = 0.0001) and 24-h urinary Na (P = 0.0001) as expected. Indomethacin significantly decreased mean heart rate (P = 0.018). The effects of NaCl and I on SBP, DBP and heart rate were not modified by age, alcohol intake, serum K+, body mass index or treatment order. In the ISH group, NaCl dose significantly elevated SBP (9.87 mm Hg; P = 0.0001) and DBP (5.26 mm Hg, P = 0.006) but did not significantly alter blood pressure in the normotensive group. Indomethacin significantly elevated SBP (P = 0.03) in normotensive individuals but had no effect on blood pressure in the ISH group.

Conclusions: Chronic high salt diet elevated blood pressure more than I in the total cohort of elderly individuals. No interaction was demonstrated and their effects were additive. In the ISH group, chronic high salt diet significantly increased SBP and DBP while I failed to alter blood pressure. In the normotensive group, I, but not salt, elevated SBP. Patients with ISH are sensitive to the pressor effect of NaCl but resistant to the pressor effect of prostaglandin inhibition in contrast to elderly normotensive control individuals where the reverse was found.