The resistance in a blood vessel is equal to the pressure difference divided by the blood flow. Let us now say that a sympathomimetic causes vasoconstriction which increases the resistance. Does this primarily increase the pressure in the blood vessel? Does it primarily reduce the blood flow in the blood vessel?
In context, the filtration fraction of the kidneys is the GFR divided by the plasma blood flow through the afferent arteriole. The sympathomimetic would in general increase the filtration fraction by reducing blood flow. However, the GFR is also dependent on hydrostatic pressure which may increase if there is a rise in resistance. Therefore, shouldn't the filtration fraction change minimally due to the rise in GFR counteracting the fall in blood flow?