Role of intramitochondrial nitric oxide in rat heart and kidney during hypertension

Abstract

Nitric oxide (NO) is an important reactive molecule in many organisms. A mitochondrial nitric oxide synthase has been described; however, the role of NO in this organelle is not yet fully clear. We tested the effect of intramitochondrial NO on various functions from spontaneously hypertensive rats (SHR) and their normotensive genetic control, Wistar–Kyoto (WKY) rats. While the stimulation of intramitochondrial NOS increased calcium- and phosphate-induced permeability transition pore opening, its inhibition partially prevented it, without affecting membrane potential. Matrix free calcium and the pH decreased with NOS inhibition. Basal [NO] was lower in SHR than in WKY. Our data suggest that intramitochondrial NO plays an important role in mitochondrial regulation during hypertension.

Introduction

Nitric oxide (NO) is an ubiquitous molecule from bacteria to mammals. Some of the most recent fields in NO research are hypertension and apoptosis (Hu et al., 1994, Chou et al., 1998, Qiu et al., 1998); for instance, it is known that NO is one of the most powerful vasodilators produced by the endothelium and it is deeply involved in vascular pathologies, such as hypertension (Moncada et al., 1991, Ignarro et al., 1999). This phenomenon is associated with a diminished activity of the endothelial nitric oxide synthase (eNOS) (Shesely et al., 1996, Kurihara et al., 1998, Miyamoto et al., 1998). In support of this contention, it has been demonstrated that inhibition of NO production by the endothelium using analogs of l-arginine (l-arg) produces a significant increase in blood pressure (Tabrizchi and Triggle, 1991, Hu et al., 1994, Mattson and Bellehumeur, 1996, Qiu et al., 1998, Sandner et al., 1999).

NO is a free radical synthesized by the nitric oxide synthase (NOS) (Cooke and Dzau, 1997, Mayer and Hemmens, 1997, Cannon, 1998). The substrate of this enzyme is the amino acid l-arg and its activity is inhibited by analog molecules such as N-nitro-l-arginine methyl ester (l-NAME) and N^ω-monomethyl-l-arginine (l-NMMA) (Boucher et al., 1999, Tsikas et al., 2000). Recently, several groups have presented evidence that constitutively expressed and continuously active NOS was found in mitochondria (Bates et al., 1995, Ghafourifar and Richter, 1997, Tatoyan and Giulivi, 1998, López-Figueroa et al., 2000, French et al., 2001, Manzo-Ávalos et al., 2002). The mitochondrial nitric oxide synthase (mtNOS) is attached to the matrix face of the inner membrane and its catalytic activity is Ca²⁺ dependent (Ghafourifar and Richter, 1997, Tatoyan and Giulivi, 1998). To date, there are limited data available on the subcellular role of nitric oxide in hypertension. The purpose of this study was to gain further insight into the potential role of the intramitochondrial NO synthesis in the regulation of mitochondrial function in rat heart and kidney during hypertension. To aid in the interpretation of the data derived from the experiments, we report the effects of hypertension on in vitro preparations of viable mitochondria.