Regulators of the cardiovascular system and, in particular, in the treatment of hypertension having the formula ##STR1## pharmaceutically-acceptable bioprecursors thereof, and the pharmaceutically-acceptable acid addition salts thereof; wherein each of R.sup.1 and R.sup.2 is C.sub.1-4 alkyl or --CH.sub.2 CF.sub.3 ; alk is ethylene, monophenyl substituted ethylene, mono- and dimethyl substituted ethylene or monophenylmonomethyl substituted ethylene; and R.sup.3 is hydrogen, C.sub.1-4 alkyl, C.sub.3 -C.sub.6 cycloalkyl; phenyl or substituted phenyl; and methods for their preparation.