Roles of nitric oxide synthase isoforms in cutaneous vasodilation induced by local warming of the skin and whole body heat stress in humans

Abstract

Nitric oxide (NO) participates in the cutaneous vasodilation caused by increased local skin temperature (Tloc) and whole body heat stress in humans. In forearm skin, endothelial NO synthase (eNOS) participates in vasodilation due to elevated Tloc and neuronal NO synthase (nNOS) participates in vasodilation due to heat stress. To explore the relative roles and interactions of these isoforms, we examined the effects of a relatively specific eNOS inhibitor, N(omega)-amino-l-arginine (LNAA), and a specific nNOS inhibitor, N(omega)-propyl-l-arginine (NPLA), both separately and in combination, on skin blood flow (SkBF) responses to increased Tloc and heat stress in two protocols. In each protocol, SkBF was monitored by laser-Doppler flowmetry (LDF) and mean arterial pressure (MAP) by Finapres. Cutaneous vascular conductance (CVC) was calculated (CVC = LDF/MAP). Intradermal microdialysis was used to treat one site with 5 mM LNAA, another with 5 mM NPLA, a third with combined 5 mM LNAA and 5 mM NPLA (Mix), and a fourth site with Ringer only. In protocol 1, Tloc was controlled with combined LDF/local heating units. Tloc was increased from 34 degrees C to 41.5 degrees C to cause local vasodilation. In protocol 2, after a period of normothermia, whole body heat stress was induced (water-perfused suits). At the end of each protocol, all sites were perfused with 58 mM nitroprusside to effect maximal vasodilation for data normalization. In protocol 1, at Tloc = 34 degrees C, CVC did not differ between sites (P > 0.05). LNAA and Mix attenuated CVC increases at Tloc = 41.5 degrees C to similar extents (P < 0.05, LNAA or Mix vs. untreated or NPLA). In protocol 2, in normothermia, CVC did not differ between sites (P > 0.05). During heat stress, NPLA and Mix attenuated CVC increases to similar extents, but no significant attenuation occurred with LNAA (P < 0.05, NPLA or Mix vs. untreated or LNAA). In forearm skin, eNOS mediates the vasodilator response to increased Tloc and nNOS mediates the vasodilator response to heat stress. The two isoforms do not appear to interact during either response.

Fig. 1.

Illustration of protocol 1. This protocol was designed to examine the effects of separate and combined blockade of endothelial nitric oxide synthase (eNOS) and neuronal NOS (nNOS) on the vasodilation induced by local skin warming. The perfusion rate at all microdialysis sites was 3 μl/min. Tloc, local skin temperature; LNAA, N^ω-amino-l-arginine; NPLA, N^ω-propyl-l-arginine.

Fig. 2.

Illustration of protocol 2. This protocol was designed to examine the effects of separate and combined blockade of eNOS and nNOS on the vasodilation induced by whole body heat stress. The perfusion rate at all microdialysis sites was 3 μl/min. AVD, active vasodilation.

Fig. 3.

Summary of cutaneous vascular conductance (CVC) responses to local skin warming. At a local skin temperature (Tloc) of 34°C, CVC values, normalized to their respective maxima as induced by 56 mM nitroprusside perfusion, did not differ significantly between perfused with Ringer solution alone, 5 mM LNAA or NPLA in Ringer, or a combination of the 2 NOS antagonists in Ringer (P > 0.05 between sites). CVC increased in response to local skin warming at all sites (P < 0.05, 34°C vs. 41.5°C). At a Tloc of 41.5°C, these increased CVC values were significantly different between those sites perfused with either Ringer solution alone or NPLA alone compared with CVC values at sites perfused with either LNAA alone or a combination of the LNAA and NPLA in Ringer (P > 0.05 Ringer vs. NPLA; *P < 0.05 Ringer or NPLA vs. LNAA or combined LNAA and NPLA; P > 0.05 LNAA vs. combined LNAA and NPLA). Thus LNAA treatment attenuated the CVC response to local skin heating, but NPLA treatment added no further attenuation.

Fig. 4.

Summary of CVC responses to whole body heat stress. In normothermia, CVC values did not differ between sites (P > 0.05 between sites). In response to whole body cold stress, CVC fell at all sites (P < 0.05). These responses did not differ between sites (P > 0.05 between sites). During whole body heat stress, CVC increased at all sites (P > 0.05). At the peak of heat stress, CVC values were significantly different between those sites perfused with either Ringer solution alone or LNAA alone compared with CVC values at sites perfused with either NPLA alone or a combination of the LNAA and NPLA in Ringer (P > 0.05 Ringer vs. LNAA; *P < 0.05 Ringer or LNAA vs. NPLA or combined LNAA and NPLA; P > 0.05 NPLA vs. combined LNAA and NPLA). Thus NPLA treatment attenuated the CVC response to local skin heating, but LNAA treatment added no further attenuation.