<PAGE>   1
                                                                     EXHIBIT 4c2

                             SUPPLEMENTAL INDENTURE


         This Supplemental Indenture, dated as of August 6, 1997 (this
"Supplemental Indenture"), among Houston Lighting & Power Company, a Texas
corporation ("HL&P"), HI Merger, Inc., a Delaware corporation ("HI Merger"),
NorAm Energy Corp., a Delaware corporation and successor in interest to Arkla,
Inc. (the "Company"), and Citibank, N.A., as Trustee (the "Trustee"),
supplements the Indenture dated as of April 15, 1990 (the "Indenture") between
the Company and the Trustee.

                                    RECITALS

                 WHEREAS, pursuant to an Agreement and Plan of Merger dated as
of August 11, 1996, as amended by the Amendment to Agreement and Plan of Merger
dated as of October 23, 1996 (the "Merger Agreement"), among Houston Industries
Incorporated, a Texas corporation ("HI"), HL&P, HI Merger and the Company, HI
will be merged with and into HL&P, with HL&P to be the surviving corporation
and renamed "Houston Industries Incorporated" ("Houston"), and the Company will
be merged with and into HI Merger (the "NorAm Merger"), with HI Merger to be
the surviving corporation and renamed "NorAm Energy Corp.," as a result of
which each outstanding share of NorAm Common Stock will be converted into
either cash or shares of common stock, no par value, of Houston ("Houston
Common Stock"), as set forth in the Merger Agreement;

                 WHEREAS, in connection with the NorAm Merger, HL&P, the
Company and HI Merger have duly determined to make, execute and deliver to the
Trustee this Supplemental Indenture in order to reflect the results of the
NorAm Merger as required by the Indenture;

                 WHEREAS, pursuant to Section 8.01 of the Indenture, HI Merger,
as the surviving corporation of the NorAm Merger, is required to expressly
assume, by an indenture supplemental to the Indenture, the due and punctual
payment of the principal of (and premium, if any) and interest on all the
Securities and the performance of every covenant of the Indenture on the part
of the Company to be performed or observed; and

                 WHEREAS, Section 9.01 of the Indenture provides that under
certain conditions the Company and the Trustee, without the consent of the
Holders of Securities, from time to time and at any time, may enter into an
indenture supplemental to the Indenture, inter alia, to evidence the succession
of another corporation to the Company and the assumption by any such successor
of the covenants of the Company under the Indenture, and in the Securities;
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                 NOW, THEREFORE, THIS SUPPLEMENTAL INDENTURE WITNESSETH:

         In consideration of the premises and other good and valuable
consideration, the receipt and sufficiency of which are hereby acknowledged,
and to comply with Sections 8.01 and 9.01 of the Indenture, the parties hereto
hereby agree, for the equal and proportionate benefit of the respective Holders
from time to time of the Securities, as follows:

         Section 1.    Defined Terms.  Capitalized terms used and not otherwise
defined herein have the respective meanings assigned to such terms in the
Indenture.

         Section 2.    Succession by Merger.

         (1)     As of the effective time of the NorAm Merger, HI Merger shall
become the successor to the Company for all purposes of the Indenture and HI
Merger hereby expressly assumes the due and punctual payment of the principal
of (and premium, if any) and interest on all the Securities and the performance
of every covenant of the Indenture, as supplemented by this Supplemental
Indenture, on the part of the Company to be performed or observed.

         (2)     Concurrently with the execution and delivery of this
Supplemental Indenture, the Company has delivered to the Trustee an Officers'
Certificate and an Opinion of Counsel as required by Sections 1.02 and 8.01 of
the Indenture.

         Section 3.    Ratification.  The Indenture, as hereby supplemented is
in all respects ratified and confirmed by each of the parties hereto, and all
of the rights and powers created thereby or thereunder shall be and remain in
full force and effect.

         Section 4.    Governing Law. The laws of the State of New York shall
govern this Supplemental Indenture without regard to principles of conflict of
laws.

         Section 5.    Successors.  All agreements of the parties hereto in
this Supplemental Indenture shall bind their respective successors.

         Section 6.    Multiple Counterparts.  The parties hereto may sign
multiple counterparts of this Supplemental Indenture.  Each signed counterpart
shall be deemed an original, but all of them together represent the same
agreement.





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         IN WITNESS WHEREOF, the undersigned have caused this Supplemental
Indenture to be executed by its duly authorized officer as of the date first
above written.


                                     HOUSTON LIGHTING & POWER COMPANY


                                     By: /s/ HUGH RICE KELLY
                                        -----------------------------------
                                        Name:  Hugh Rice Kelly
                                        Title: Executive Vice President, 
                                               General Counsel
                                               and Corporate Secretary

Attest:


By:  /s/ RUFUS S. SCOTT
   -----------------------------------
   Name:  Rufus S. Scott
   Title: Assistant Corporate Secretary


                                     HI MERGER, INC.


                                     By:  STEPHEN W. NAEVE
                                        -----------------------------------
                                        Name: Stephen W. Naeve
                                        Title: President

Attest:


By:  /s/ RICHARD B. DAUPHIN
   -----------------------------------
   Name:  Richard B. Dauphin
   Title: Assistant Corporate Secretary





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                                     NORAM ENERGY CORP.


                                     By:  /s/ T. MILTON HONEA
                                        -----------------------------------
                                        Name:  T. Milton Honea
                                        Title: President

Attest:


By:  /s/ HUBERT GENTRY, JR.
   -----------------------------------
   Name:  Hubert Gentry, Jr.
   Title: Secretary

                                     CITIBANK, N.A.
                                     as Trustee


                                     By:  /s/ CAROL NG
                                        -----------------------------------
                                        Name:  Carol Ng
                                        Title: Vice President

Attest:


By:  /s/ ARTHUR W. ASLANIAN
   -----------------------------------
   Name:  Arthur W. Aslanian
   Title: Vice President





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