The goal of this project is to determine the genetic factors contributing to interindividual differences in response to physiological and pharmacological vasoconstrictors and vasodilators.

The dorsal hand vein model is a relatively non-invasive and robust experimental model to examine the local in vivo effect of vasoactive drugs without elucidating systemic counterregulatory reflexes. Infusion of incremental low doses of phenylephrine into a dorsal hand vein results in increasing local venoconstriction, without systemic effects. Similarly, infusion of incremental low doses of nitroglycerin into a preconstricted dorsal hand vein results in increasing local venodilation, without systemic effects. Systemic vascular responses can be measured by the cold pressor test (CPT),that leads to increase in blood pressure and heart rate , or mental stress that is also known to stimulate cardiovascular responses. Individuals vary in their local and systemic vascular responses but the genetic determinants of these are not clear.

Inclusion Criteria:

• Age between 18 and 40 years, inclusive.
• Subject must be willing to give written informed consent and be able to adhere to diet and study schedules.
• Subjects must be free of any clinically significant disease that requires a physician's care and/or would interfere with the study evaluations.
• Subjects must have a normal or clinically acceptable physical examination and ECG.
• Clinical laboratory tests (CBC, blood chemistries, and urinalysis) must be within normal limits or clinically acceptable to the investigator.

Exclusion Criteria:

• Any subject who has taken any prescription or over-the-counter drugs, other than oral contraception if female, within two weeks prior to study drug administration.
• Subjects who are presently, or were formerly, narcotic addicts or alcoholics.
• Subjects who have a clinically significant allergy/intolerance to phenylephrine.
• Females with a positive serum/urine pregnancy test at screening.
• Females who are nursing.
• Subject using sildenafil or other phosphodiesterase inhibitors.

Adverse events were documented for participants from time they began diet of at least 4 days to end of study day.

Findings derived from dorsal hand veins and cannot automatically be extrapolated to other vascular beds.

Women were not studied at predefined periods of menstrual cycle or account for use of hormonal contraceptives.

Some genotype groups were small.