Synthesis and pharmacology of new 1,4-dihydropyridines. 2,6-Dimethyl-4-(substituted phenyl) or (2-furyl)-, (2-thienyl)- or (3-pyridyl)-3,5-di[(N-methyl) or (N-diethyl)]carbamoyl-1,4-dihydropyridines as potent calcium-channel blockers