The effects of melatonin on blood pressure and heart rate were studied in 23-week-old male spontaneously hypertensive rats. Melatonin infused i.p. at a dose of 6 mg/rat per day for 5 days using an osmotic minipump produced a significant reduction of blood pressure and a slight but significant decrease of heart rate in the conscious and unrestrained state. These cardiovascular effects of melatonin developed gradually. Plasma renin concentration tended to decrease after melatonin treatment. These results demonstrate that melatonin has an antihypertensive action. The mechanism of the antihypertensive action of melatonin requires further study.

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: Acetylcholine (ACh) is present in human blood and localized in T-lymphocytes which exhibit ACh-synthesizing activity. As the final step towards clarifying the origin of ACh present in the blood, we elucidated the enzyme involved in ACh synthesis in T-lymphocytes by detecting the mRNA for choline acetyltransferase (ChAT), which is known to catalyze ACh synthesis in the nervous system. The ChAT mRNA in the MOLT-3 human leukemic T-cell line was amplified by reverse transcriptionpolymerase chain reaction (RT-PCR) using specific order and reverse primers. A single specific RT-PCR product was observed on agarose gel. The sequence of the RT-PCR product of MOLT-3 was completely identical with nucleotide positions 322-973 of human brain ChAT cDNA. Western blot analysis with an antibody to ChAT confirmed the presence of ChAT protein in MOLT-3. These findings demonstrate that mRNA for the same ChAT as that in the nervous system is expressed in T-lymphocytes, and indicate that ACh synthesized by ChAT in T-lymphocytes is the origin of ACh in the blood.