Table 2.

Effects of nNOS inhibition on HR responses (A, beats min⁻¹) or interbeat interval changes (B, ms) to vagal nerve stimulation in isolated atria in vitro

A.	Wild-type						nNOS+/−
	−EX			+EX			−EX			+EX
	Con	Ad.eGFP	Ad.nNOS	Con	Ad.eGFP	Ad.nNOS	Con	Ad.eGFP	Ad.nNOS	Con	Ad.eGFP	Ad.nNOS
Control
3 Hz	50	44	123	99	76	126	58	60	139	48	55	137
	± 5	± 4	± 12*	± 11**	± 3**	± 8*	± 8	± 10	± 36*	± 8	± 3	± 10*
5 Hz	89	82	159	112	125	192	86	92	153	85	92	202
	± 6	± 5	± 20*	± 11**	± 8**	± 13*	± 7	± 10	± 35*	± 16	± 3	± 12*
l-VNIO
3 Hz	44	31	59	53	51	59	51	40	40	39	50	57
	± 5**	± 8**	± 9**	± 17**	± 4**	± 8**	± 7**	± 5**	± 10**	± 7**	± 6**	± 5**
5 Hz	66	53	78	67	81	86	78	84	75	70	76	91
	± 12**	± 17**	± 17**	± 10**	± 4**	± 7**	± 4**	± 11**	± 23**	± 13**	± 5**	± 7**
l-Arg
3 Hz	48	62	107	96	82	98	55	59	108	53	58	96
	± 3	± 11	± 5*	± 13**	± 11**	± 6*	± 4	± 6	± 24*	± 10**	± 5	± 17*
5 Hz	83	103	140	103	107	132	83	98	116	80	89	147
	± 5	± 10	± 40*	± 8**	± 11**	± 11*	± 7	± 8	± 20*	± 12**	± 4	± 22*
Baseline HR (beats min−1)
	376	371	378	350	349	367	383	398	388	354	379	347
	± 25	± 18	± 29	± 19	± 33	± 30	± 20	± 29	± 25	± 23	± 34	± 32
B.	Wild-type						nNOS+/−
	−EX			+EX			−EX			+EX
	Con	Ad.eGFP	Ad.nNOS	Con	Ad.eGFP	Ad.nNOS	Con	Ad.eGFP	Ad.nNOS	Con	Ad.eGFP	Ad.nNOS
Control
3 Hz	34	31	87	69	49	99	28	29	89	27	32	108
	± 2	± 1	± 5*	± 7**	± 3**	± 5*	± 3	± 4	± 16*	± 4	± 2	± 5*
5 Hz	59	54	127	82	97	210	45	49	104	54	60	226
	± 2	± 2	± 9*	± 7**	± 5**	± 8*	± 3	± 4	± 15*	± 9	± 2	± 6*
l-VNIO
3 Hz	31	24	39	32	31	36	24	18	18	21	28	33
	± 12	± 13	± 13†	± 10†	± 3†	± 5†	± 3	± 2†	± 4†	± 4†	± 3†	± 3†
5 Hz	44	36	51	42	53	57	40	44	38	42	47	59
	± 15	± 17	± 17†	± 6†	± 3†	± 5†	± 1†	± 4	± 10†	± 7†	± 3†	± 4†
l-Arg
3 Hz	32	41	73	66	54	68	26	28	61	30	34	64
	± 1	± 4	± 2*	± 8**	± 7**	± 4*	± 1	± 2	± 10*	± 5†	± 3	± 9*
5 Hz	55	70	104	73	77	105	43	54	68	50	57	121
	± 2	± 4	± 2*	± 5**	± 7**	± 7*	± 3	± 3	± 8*	± 6†	± 2	± 12*

In vitro atrial beating rate responses to vagal nerve stimulation (VNS) at 3 and 5 Hz stimulation frequencies. Part A shows rate responses calculated as beats per minute change and part B shows responses expressed as change in contraction interval (ms). Control responses were significantly increased in trained (+EX) compared to untrained (−EX) in wild-type tissue

^(**P < 0.05, unpaired t test), although this effect was not present in atria from mice lacking one allele of neuronal nitric oxide synthase (nNOS^+/−). Gene transfer using adenoviruses containing nNOS (Ad.nNOS) increased responses to VNS in all atria compared to gene transfer using adenoviruses containing enhanced green fluorescent protein (Ad.eGFP) and control (Con, no gene transfer) atria

^(*P < 0.01, unpaired t test). Selective nNOS inhibition with vinyl-l-N-5-(1-imino-3-butenyl)-l-ornithine (l-VNIO, 100 µmol l⁻¹) attenuated the response to nerve stimulation in all atria, and abolished the effects of training and Ad.nNOS treatment. This effect was reversed in all cases with l-arginine l-Arg, 1 mmol l⁻¹,

^†P < 0.05, repeated measures ANOVA.