Table 1. Intracellular retinoic acid concentrations following incubation with or without 50 μM acitretin for 24 h prior to the addition of 10 μM ATRA or 13cisRA for a further 24 ha.

		ATRA incubation (μM)			13cisRA incubation (μM)
Cell line	RA isomer	Control	Acitretin	P b	Control	Acitretin	P b
SH-SY5Y	ATRA	39.1±7.4	113.5±28.7	0.003	4.2±1.1	19.3±4.1	0.005
	13cisRA	7.6±1.3	7.5±0.7	0.744	25.4±4.1	28.7±6.9	0.674
SH-EP	ATRA	908.1±378	1050±439.3	0.652	49.9±10.6	51.9±18.2	0.933
	13cisRA	43.5±9.1	39.9±11.3	0.886	92.7±19.7	75.7±29.1	0.435
IMR-32	ATRA	354.6±31.6	447.9±83.6	0.382	59.9±31.1	120.2±66.2	0.707
	13cisRA	29.7±2.5	26.5±1.6	0.352	39.4±17.3	60.4±14.6	0.378
NGP	ATRA	613.3±165.3	1657±688.3	0.083	32.5±15.2	21.4±1.4	0.642
	13cisRA	46.0±11.7	67.2±23.4	0.41	84.6±11.4	106.9±7.7	0.221

^aResults are expressed as mean±s.e.m. from n⩾3 experiments.

^bProbabilities from two-sided t-tests.