The influence of an external orienting field on isostructural transformations taking place in the stability area of smectic state is studied on the basis of a broaden version of the de Gennes model of smectic A (Sm) liquid crystal. It is predicted that the external field can suppress the isostructural Sm-1-Sm-2 phase transition in some systems and, conversely, it can induce the isostructural smectic state in other ones. The most unexpected case includes the situation when both phenomena can be realized in one and the same system, so that two Sm-1-Sm-2- paranematic points take place in the "field-temperature" phase diagram simultaneously.